Looks like a result of several rare factors which unfortunately happened all at once for the passenger. Bumped head in the wrong spot = broken neck? Unbalanced curtain airbag inflation? Or maybe failed curtain airbag detonation? I see remains of all units except the passenger side unless it was cut right away.
Strangely enough I was in the middle of mailing out a pass. curtain airbag for a f10 m5.