It's very likely the front viewport failed, causing the entire vessel to depressurise and instantly crumple, killing everyone inside instantly. Chances of survival are improbable at best
Update: The crushed remains of the submersible were found floating on the surface. Evidently the submersible's mothership and possibly U.S. Navy hydrophones heard the sound of the sub's implosion before the search and rescue mission started. But this fact was not made public until after the remains were identified.