Indeed, some of the acts must have been motivated more from the Quran than anything else. This could probably be said of the Bible and various other texts as well. One question is the proportion for each various classification of root cause. Something tells me that the greater the size of the group acting in a particular instance, the less likely it is to be motivated merely by the Quran. There are plenty of instances of individuals in the West having unusual religious and or cult-like beliefs who then go on to act in violence out of those beliefs.
As far as susceptibility, I would imagine it's pretty much the same as for all sorts of other fringe groups, such as the KKK. Sometime individuals join such groups to give themselves a sense of brotherhood, which is probably more tempting for individuals who don't fit into other circles of society for one reason or another.
Perhaps what makes these Islamic extremist groups different from many others is that (a) they are often completely underground and (b) they actively (and successfully) recruit people who may be susceptible. My guess is this active recruitment (a) makes the groups bigger, (b) provides them with the tools they need, and (c) gives the participants an assurance that they will be remembered for helping the group in its goals.
Back to the original point, I'm thinking that those at the top of these groups are like politicians in Western states -- they issue plans and orders to those below them, who don't necessary know what the true cause behind the acts is. By appealing to a higher power (the Quran), the higher-ups are able to obscure the true reasons behind the plans.
The true reasons are probably known only at the top. Any grievances between groups of people could be included in these reasons, whether over resources, dominance, influence, ethnicity, land, or culture. I guess what I was trying to say from the beginning is that the primary drive is probably one of the more common reasons listed above, while the official justification involves the scriptures. Few groups of people in this world, it seems, will admit that they are acting out of self-interest. A more popular approach seems to be saying it was a higher power's wishes.
A good counterargument might be that these people are acting in violence to gain their reward(s) in the afterlife. The Bible too calls for violence in various instances, to my understanding, but it sounds like the Quran is more explicit about the rewards for violence. Nevertheless, people are tricksters. It would not surprise me if the higher-ups are using these rewards to drive those at the bottom to carry out acts toward a defined worldly goal. After all, then there would be benefit both for the higher-ups and for the suicide bombers. Being able to convince others that they will receive grand rewards in the afterlife if they just carry out your plans would be a very powerful tool.