I've had brakes seize on during storage a couple of times. Quite why this happens when the hand brake is off, I'm not sure.
However, it is very easy to free them off if you do it the right way: jack the wheel clear of the ground and tap the back of the drum with a hammer. It only takes a couple of minutes.
Dragging the trailer, hitting it anywhere else, trying to avoid jacking it up, will not work (and is the route to madness).
I learnt this lesson the hard way a couple of years ago. After spending two hours dragging, kicking and hitting the trailer in all directions, once I bothered to get the jack out it was all over in moments!