Gravity. The planes/explosives just gave it a little push. And don't think that BOTH towers 1 & 2 falling is a coincidence - just good planning.
Saw some more footage up close to the towers as they collapsed. It was a docodrama about 9-11. The floors are popping out OBVIOUSLY because of explosive charges.
It took 20+ years to fully expose Kennedy's assassination. Look on the bright side...9-11 was pulled apart from day 1.