Historically, the root cause of terrorism is always the same...passion for a cause + greatly disproportionate resources that makes it impossible to fight for the cause in a conventional manner = terrorism.
With the occasional exception of state-sponsored terrorism that is employed as a tactic in a broader campaign and is inspired by true terrorism but lacks the idealism and passion. Perhaps instead of terrorism, we should call it malignant desperationism. Then, perhaps, when we asked what makes a person, a group, malignantly desperate we could gain some insight.
Approaching the topic from a historical perspective, though, I often wonder what caused the shift in the late 20th century away from the highly effective methods of peaceful resistance and nonviolent civil disobedience and toward the much less effective tactic of terrorism. I believe the shift occurred in the 70's, and it seemed to be a sudden seismic shift that began in the Middle East but was taken up by all resistance groups by the 80's and continuing through today.