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.