Looks like a gut rehab without permits. So it appears to have been an inside job The first floor corner wall collapses vertically, which is strange but the fence hides what's happening. The fire engine is a weird coincidence; it was returning to the station which is literally 1 block away.
lol @ the absolutely epic firefighter response time
(And yes, it's incredibly obvious they were about to pass in front of that building anyway, and it just collapsed right in front of them so they stopped)