RUMOR 1
Second hand information: something in the pitot static system iced up so the pressure to compare against by the pressurizing system returned a wrong value hence the pressure kept on increasing as the comparison value never got reached since the signal to the switch never matched up to get the pressurizing to stop.
As I said, second hand info so possibly missing some important detail.
RUMOR 2
Also heard that the aircrew were in bad condition due to something like the bends happening to them all that pressure, sort of like rapid depth increase in a submarine or scuba diving, then all released at once.
As noted, all above RUMOR and from a second hand source. Plausible, but bits missing.