My money’s on a homeless encampment.
That was my first thought too — started by homeless squatters. In my old Chicago neighborhood, nestled among the nice residential homes, were several small, old, abandoned “factories” where the homeless squatted and pursued their shenanigans. Late one night one of the places caught fire. The homeless folks just up and left, notifying no-one. By the time someone noticed the flames, several homes were lost. The “homeless” are a plague.