1. Fire.
   
2. Nothing was strong enough in the structure to act as a hinge to lever 30 or 40 floors laterally, so they just fell downward.
   
3. I can't have an opinion on this, because I simply cannot grasp the forces involved when that much mass is no longer supported. I don't think anyone beyond a civil engineer can.
4. Two buildings were hit the same way and failed the same way. If anything, that shows consistency. The failure of floor trusses is such a known problem that in many municipalities buildings are required to post signs to warn firefighters. As for building 7, it probably would have stayed up if it wasn't for the odd construction to accommodate the con-ed substation. None of them were "fortified".
   

One personal observation I have is if these buildings fell from demolition charges, why didn't I and the millions of other people watching within a few square miles hear them? Every video of controlled demolition I've ever seen you can hear the charges going off for quite a distance. The very nature of an explosion that is going to take out a support beam would seem to require enough blast pressure to create a sound wave that millions would have heard. While I didn't see building 7, I watched tower 1 and 2 fall with my own eyes and I can tell you there were no explosions prior to the rumbling.