@ temp spike : The armchair enthusiasts at it again - deciphering the great red light mysteries from seat 7C and declaring the crew are incompetent idiots.
SAA has some of the best trained pilots in the world and they would have assessed the situation and tried to correct it according to the QRH. Sometimes there are failures which occur which are very uncommon and this is when experience comes to the fore and luckily, SAA has some of the most experienced crews on the continent. During the assessment, they would weigh up the severity of the failure/malfunction and decide to continue to destination, return to the departure field or carry out an en-route diversion. Part of their procedures includes a briefing to the cabin crew face to face and this is when the cockpit door was probably opened. As for worrying about who closed the door, I think that was the last thing the pilots would be overly concerned about. Nothing stupid about getting the aircraft home in one piece IMHO.
As for the original question, the B737 uses bleed air for pressurisation. The bleed air comes from the engines so if one engine is malfunctioning and the thrust is all over the place then the pressurisation would become a bit erratic too. Haven't flown the -800 in years so can't be more specific about the likely cause but thats my 2c worth.