2:42 is the time of the first emergency call.
If you watch this video (
youtube.com/watch?v=6I7yJWxq93U )
you will see Ray Bowen stating that Stack collapsed at 2:28, with agrees with witness statements that Stack had been down for about 20 minutes before emergency crews got to the scene.
As far as I can tell, the reason for using 2:42 is that it is actually on record, and not simply guesstimates.