What caused this

Curious too why the MP was pulled.
Every installer needs to verify lifter rotation during assembly, plus a break in paste such as Crane's should be on the lobes and lifter faces, plus the engine should only be turned over to attach the flex plate to the convertor, and to start it. If you crank for oil pressure, and crank for fuel pressure, and crank to set the distributor you are asking for a failure these days.