You are quite correct that brakes are released as soon as you get onto stand, after the aircraft is chocked. This is to allow the brakes to cool down, during the turnaround, they normally only weld together, if very hard braking has been applied, in the case of an RTO. Parking brake pressure will normally dissipate over a long length of time, which is why the parking brake and chocks are applied overnight and in windy conditions.
However aircraft will swing around, as happened on this occasion even if chocked and the parking brake has been applied, if they are not pointed into wind. This is why, if high winds are forecast that the airlines get their engineers to park the aircraft into wind. It is also not unusual for the aircraft to be fuelled to add weight also.
This is one of the problems that airlines face when operating from large airports, it is very difficult to attach a bridge to an aircraft, if it is parked at 90 degrees from the centre line.
I wonder what would happen if the likes of BA, BMI and Virgin told the Heathrow Duty officer, that they wanted to turn their aircraft into wind on a particular day.