Apparently the viewport was rated for 1300m, and they were driving to 4000m. The fact it survived as long as it did is testament to the manufacturer's standards. The fact that it failed is utterly unsurprising.
Also the inspector they hired to verify the sub's safety was denied when he requested equipment to scan and test the hull integrity, was fired when he raised these concerns, and was sued for leaking company secrets when he tried to report it to OSHA.
Honestly the only surprising part is that it survived the previous thirteen dives before this one.
It restarts at random.