On my car I press the unlock button once to unlock just the drivers door, and again to unlock the whole car. The last couple of days, when pressing the unlock button twice to get something off the back seat it opened the tailgate. I only noticed by accident the first time taking the dog out at night, walking past the car seeing out the corner of my eye the light on the dash to say the boot was open.
It has since stopped doing this. I have no idea why. Cars can be very fickle.