Most likely your mast is bowed slightly. Same thing happened on my 2000 Hobie 16. Mast developed just a 3-4" deflection after sailing in really heavy winds. Never flipped but when came back to beach crew noticed the bend. Tried few times to straighten mast by bouncing on it, but bow always came back. Mast wouldn't rotate to one side consistantly which was a real pain in races. Finally had to take an old straight mast, put a comtip on it. Since then no problems with rotating.
Good Luck,
Stew