Revenge was one of the causes for war:  "Iroquoian conflicts traditionally arose when the families of dead warriors demanded satisfaction. The conflicts could well smoulder quietly for a certain amount of time before erupting in a series of raids, attacks and counterattacks, all seeking revenge, with the last attack always justified by the previous one. In this way, a climate of virtually incessant hostility and violence was perpetrated between the various nations. Decisions to raise war parties could also be prompted by the dreams of chiefs or war priests, falsely called sorcerers by the white men."  (Source)

Another cause was a traditional suspicion of other cultures:  When considering Lowland Cree--Inuit warfare it was found that  "such conflict predates the arrival of European colonials," Though indeed, "a post-contact change in the patterns and sources of warfare [has] been identified in those contexts.  [...] From a Cree perspective, warfare had little to do with questions of territoriality.... the prime motives for warfare entailed mutual mistrust attributable to irreconcilable ethnic differences, as well as arising from Lowland Cree misfortunes born of post-contact ecological change attributed to Inuit sorcery.“  (Source) 

In other words, in some regions, climatic changes or perhaps a depletion of the land's resources may have caused tensions.  "Inuit expansion many hundreds of years ago along the shores of the bays, a movement perhaps precipitated by changing climatic conditions and a search for resources, may have been a primary cause of the prehistoric warfare.” (Source) 

This kind of reason partially resembles the way southern agricultural peoples, moving to new fertile lands might have entered into conflicts, though movement because of new trade relations was also a factor:  “Traditionally, the Mohawk were swidden (slash and burn) agriculturalists; moving villages from time to time to prevent soil erosion and exhaustion. In some instances, Mohawk settlements were relocated in order to give the whole nation a better vantage point in dealing with other native nations and Europeans. Often migrations were accompanied by military alliance, and in some cases by religious conversion. The Six Nations Confederacy fought different nations between the years of 1608 and 1667. At various times, the Huron, Algonquin, and Montagnais allied themselves with the French and were at war with the Haudenosaunee.” (Source) 

League of Iroquois vs others:  Collective Responses and Trade Relationships

“In chapter 7, Dean R. Snow contends that precontact Iroquoian warfare was fueled in large part by the quest for revenge. In the sixteenth century, the League of the Iroquois was established as a collective response to intratribal hostilities. In the century that followed, the Iroquois sought trade relationships with various European colonial powers to exchange furs for guns. Such trade led to an increase in raiding activity by non-League nations during the course of the seventeenth century. Snow explains that such conflict heightened the demand for war captives and human trophies. This was in turn exacerbated by population losses arising from the epidemics of the time, coupled with the desire to acquire status goods and prestige items. In the eighteenth century, a surrogate for war and intertribal or intratribal conflict materialized as status and prestige could then be enhanced by the accumulation of monetary wealth and luxury goods. The rise and consolidation of power by European colonists eventually brought an end to intertribal warfare between the American Indian tribes and nations of the region.”

Source:  North American Indigenous Warfare and Ritual Violence | UAPress 

War with Iroquois in the north, even into Cree territory

““Long ago when Naataweuch [Iroquois] were still killing Cree people, a Naataweuch canoe must have tipped over near Kaa Iinuukanuuch. That’s where they found bones.

They probably thought it was safe to go down, but their canoe must have tipped and they drowned.”

- George Cannashish

To destroy the economy of New France, Naataweuch warriors attacked tribes friendly with the French. By the 1660s, their raids had extended into the Nemaska-Mistissini-Waswanipi area. Iroquoian pottery made before the raids, however, has been found along the Eastmain River. This suggests that not all contact was hostile.““

Source:  http://www.creeculture.ca/content/iroquois-raids  Archived here.

Different styles of warfare: North vs South for Eastern Woodlands peoples

“In chapter 9, George R. Milner demonstrates that Eastern Woodlands war and conflict varied over time and space and was affected by the sociopolitical milieu in which it was manifest. ... Archaic hunter-gatherers of the region fought one another with considerable frequency. [A slowdown of conflicts occurred when agricultural practices started to emerge, then there was]  ..a dramatic resurgence in armed conflict after that hiatus among the competing tribes and chiefdoms of the region. At the same time, the southern portion of the Eastern Woodlands was characterized by well-organized chiefdoms capable of mounting large-scale attacks. By contrast, in the northern portion of the Eastern Woodlands, combat was often instigated or directed by tribal headmen who conducted themselves in presumably less formally organized and certainly lower-intensity military engagements.”

Source: North American Indigenous Warfare and Ritual Violence | UAPress 

Iroquois war practices as described by Jesuits

Texts drawn from Biography of Isaac Jogues .... “Isaac was tortured by the Iroquois but forgave his tormentors and returned to preach only to die a martyr.”  ... “On the fifth day of their journey, Aug. 8, two Iroquois runners met them announcing that at a day's distance there were encamped on an island two hundred Mohawks, who were on the warpath. The Hurons knew what this signified. Indians on the warpath believe that success depends upon torture of prisoners. They also believe that by inflicting pain on others they nerve themselves to acts of bravery.”

Source: Isaac Jogues: Missionary and Martyr - Torture