Hey gang, Saturday was a great day for sailing here in west TX! The winds were 15-20 and my GF and I were out on the lake having a blast. Not many power boats proably due to the wind.
Anyway, a number of times in the past I've noticed that if I don't let out the mainsheet fast enough when coming through head-to-wind the old-windward-corner of the boat starts to submerge. I then quickly unsheet and scramble "up" to the other side and the situation is remedied. On Saturday, however, I guess I was a little slow and the boat flipped over said corner (in that case it was the port stern.)
I've given it a little thought but aside from going "okay I need to work on unsheeting when coming through head-to-wind" (and memorizing/employing Rick White's explanation of the Roll Tack) I'm not exactly sure what happened.
I'd be very appreciative if someone could tell me what's going on when that happens! Thanks in advance!
(On a sour note, the lock-nut holding my hiking stick came off so for the second half of the afternoon I was sailing just holding one tiller or the other. 
)
It's basically as you worked out, if you keep the sails cleated in, there's a chance that you go over.
If you have a leaky boat that makes it worse as all of the water in the hull goes to that corner adding to the weight in the corner.
Uncleat the mainsheet early since when you pass through the wind. Having the mainsheet cleated in tightly will not only possibly backwind and flip you as happened but will also turn you back into the wind making the turn that much harder as well. Also in strong winds, have the crew go across earlier to keep weight on that side. Get in the habit of uncleating the mainsheet before you start the turn, or better still, in heavy winds don't cleat the mainsheet at all.