Some things to note: - Obviously tons of engines were lost, could still be above the threshold required for nominal flight though. This problem might solve itself with better engine isolation/explosion protection on Booster 9. - Speculation that one of the hydraulic power units was either taken out by an engine explosion or disintegrated on its own. This would have seriously hindered Raptor gimbal and could explain the loss of control. This problem solves itself because B9 is switching from hydraulic to electric gimbal. - Fuel mixture was way off shortly after liftoff already. Seems like one of the engine/HPU RUDs maybe took out some fuel lines and caused a leak.

In the end, both stages did not separate. But what was the reason for that? - Due to insufficient thrust and/or gimbal ability, the stack never arrived at the intended hight, location and speed of stage separation. - Due to insufficient gimbal ability, the flip necessary for stage sep could not be initiated. Alternatively, if the flip is a combination between gimbal and thrust vector control, the lack of gimbal to support/counter the TVC inputs could have caused unintended and extreme maneuvering. - Some users have noted a possible buckling of the interstage because it was a weak point of the B7.1 test tank last year. I doubt the stages would have stuck together during the tumbling if that was the case, though.

Edit: Also, why did the engines keep burning during and after loss of control? I personally believe the stack was not at its intended stage separation altitude and velocity yet. Then lost control because of gimbal/HPU failure?