What is the reason behind terrorist attacks? From psychology point of view, what is the reason behind terrorist attacks?


What is the motivation for the leaders of terrorists?
What is the motivation for the person who performs terrorist attacks?
As others have mentioned, different individuals and groups can have broad (and conflicting) motivations for specific actions. I found this post on the Politics SE while looking up a similar question, though I don't know if this is exactly what you are asking. The post asks why ISIS performs terrorist attacks when it brings them more attention and threat. Ultimately, they just have different goals. Looking at it from one perspective, it seems "wrong". Looking at it from another, it completely makes sense. 

This answer (to the above question) does a great job of explaining how ISIS stated goals relate. These goals are not focused on conquest or "winning" in the traditional sense, but much more on bringing division into the world. If this is the goal, terrorist attacks that bring fear, racism, and religious judgement from the West are right (again, towards this purpose).

Long story short: they have a different goal than many people. These attacks are actually quite rational, when working towards that goal. It is the goal itself that we find irrational. 

As to why people believe in one goal vs. another: I don't think that this has a universal answer. Lots of complex things can go into this belief, and they can greatly vary by individual.