SpaceX

Scott Manley conjectures that the staging itself worked as intended, but the flip maneuver caused too much stress on the internal plumbing (with all the propellant sloshing around), resulting in a ruptured pipe and burning in the base of the booster.

From the SpaceX overlay graphic, it seems like a number of engines went out shortly after the relight for the boostback burn, around T+02:53. Asymmetric thrust and loss of control might have triggered the FTS.