3-phase motor (hi-hp) fails on start, insulation was > 500 megohms. WHY?
Welcome to Stack Exchange!  The resistance reactive fault is likely due to a short in the windings - but the short is contained within the individual phase between two adjacent turns.  This leads to a shorted turn in the winding path, with the associated high current and overheating of the winding during startup.  However, because the wire is shorted to the adjacent turn, there is no current path to the body or other windings, so the insulation from phase to phase or from phase to ground will still provide isolation and pass the hi-pot test (until the catastrophic failure).  If the short is a single turn, the change in resistance or inductance of the winding (measured at low current) will be small.