The wheels are deployed using explosive bolts, which are activated by the pilot pressing a button. The doors fly open and the wheels lock in place. (The button, BTW, is covered and well-protected, so it'd be highly unlikely that it was accidentally pushed.)
Actually the wheels are deployed by hydraulic power and mechanical bungees. They do have pyro thrusters as a backup, but I am not sure whether or not the pyros have ever been needed.