Heavy airconditioning for that very large bulding weighs tons and you can see the aircon housing on the roof collapsing just as windows popped out similarly on both sides of the building (probably pressure talong internal corridors) as floors beneathe the air conditioning began collapsing floors under their solid weight
Flashes? well my guess is if computers large battery backup was hot enough when the casings smashed as center floors collapsed onto heated equipment the volatile metals could explode on contact with oxygen and may have been the flashes
There is a vid much longer of that building collapse taken from the opposite side and gives a superior view of the top housings collapsing unevenly on the roof
My guess is the many and deep basement floors between the buildings became insecure from heat and the collapsing towers
