Maybe just some part of the construction (possibly even just the strinngers or simply some nook or cranny that is fully eclosed) got presurized or was pressurized for the whole time by just air that could not escape.

That would be fine for the fligt so far - until it started to heat up from re-entry heating. The stainless steel would be still fine if heated to hundreads of degrees, but the expanding gass could maybe make the enclosed volume to rupture ?

Or a mix of methane and oxygen accumulating somwhere and exploding - but that seems less likely to me in a near vacuum environment during re-entry.

IIRC they have pressure vessels in the lower fins with some of the gasses they need. Maybe one of those was damaged and burst. To me it looked like something blew out the bottom of one of the fins (maybe got too hot) and hit the skirt.