Ice clogged the flap root pipe and trapped residual propellant expanding rapidly from reentry warming caused it to rupture
Ice clogged the flap root pipe and an overpressure event caused it to rupture
Too much oxygen and methane mixing in an enclosed space and igniting. Clogged pipes and or solid lumps of oxygen or methane being involved in causing those excessive quantities
A loose heat shield tile struck the outer flap area
