I say V/P collision from valve float or something momentarily siezed from inadequate clearance. All pieces in related assy's are different (clearances/yield points etc) & will act different under fire. For a start I would check V/P clearance on ANY cyl. Something stacked em up solid (K.I.S.S. = vp collision)

live every 24 hour block of time like it's your last day on earth