Okay, so why do YOU think they have become terrorists?
Take a look at terrorist groups.
The IRA. They exist because of the treatment of Catholics in Ireland for as far back as Cromwell and further back. Even in recent years they were second in line for jobs, treated like dirt by the govt, ie, the British govt.
ETA. Well, the genocide in Spain from the dictator Franco was immense. Languages were prohibited, local culture was prohibited, immigration to areas, especially Catalonia and the Basque Country meant these areas were being diluted of their local population etc.
Chechen rebels. Well the USSR split up and many different countries left a country that had been non-religious but was becoming more Orthodox after many years. The Chechens wanted to leave but had their main city and area almost totally destroyed.
I could go on, but there's a pattern.
The pattern also fits for Islamic terrorists. However in this case it's on a MASSIVE scale and involves, in many cases, the west coming into Muslim countries and taking what they feel like. Iraq was made by the British, Israel has played its part, made by the British, other countries saw leaders removed, like Egypt, Iran and others, others saw troops making sure the resources were able to be taken effectively, like in say British India or many other places.
The support for Israel which has really treated people badly (this isn't a discussion of what Israel has had to endure, I'm not looking at this one sided, but we're talking why Muslims have risen up here) with millions in camps in no man's land and unable to even get on with their lives and ignored by the west.
Then the US came along, supporting Israel among other things which led to things like the bombing of the USS Cole and then 9/11. Post 9/11 and the massive increase in terrorism has had to do directly with the invasion of Iraq (more so than Afghanistan, but that played a part) and the absolutely ridiculously bad attempt at nation building by Bush (if that's what he actually wanted, I'm not so sure, perhaps he wanted and needed hatred from Muslims of the US for his own policies and party).
There's so much there that would annoy anyone enough to risk their lives.
Imagine the US had a Muslim country come in, plant troops, take resources, then later on another came along and changed your leaders so they were puppets and then invaded your country for cheaper resources. Are you telling me most people would not have risen up with their arms and fought back? Of course they would have.
So why do you expect Muslims to take this shit lying down?