Why would our front door have opened by itself? It was locked. It has an entrance set - a latched, not deadbolt, knob handled, keyed on the outside and turnsnib on the inside style lock. We have had a 40 degree Celsius day and the aircon had been on all evening and there is a bit of wind outside. The door opens inwards.
Because although it was locked, the latch was not fully engaged. If it was very cool inside and very hot outside, especially if there was any humidity, I'm not surprised to hear that the latch didn't click into place the last time it was closed.