There are little flashes happening before both pieces go down but they are probably electrical wiring being torn apart causing sparks and flashes.
Just poorly written.
If you take one of the Tweets showing the surveillance camera video and repeatedly hit the pause button from the beginning — you might have to try several times — you can see the collapse has resulted in the top floor dropping by about 1 story before the flashes start; AND there are flashes both on the top floor, and two levels below that.