According to SpaceX, the most probable root cause for the test's premature end was a "harmonic response" several times stronger in flight than had been seen in previous testing. The result was ...

“This aft section, commonly referred to as the attic, is an unpressurized area between the bottom of the liquid oxygen tank and the aft heat shield. Sensors in the attic detected a pressure rise ...

Fires in the aft section of Starship, located between the bottom of its liquid oxygen tank and rear heat shield, caused “all but one of Starship’s engines to execute controlled shut down ...