So... there is this one guy who I roll with a fair bit, and I outweigh him by about 15lbs, so often find myself on top.
Anyway, when I pass his half guard to side control he hooks his far leg the whole way over my body and gets a hook in. This leads to a slightly weird position where he basically has my back, but I have the underhook/cross face.
Eventually he works his head out or there is a scramble and he takes my back.
Any tips to stop this from happening?
Sounds like you're giving him too much space when you pass.
To get his leg around you like that, he need to turn into you and lay on his side/hip. Keep pressure on his chest and hip when you pass, so he'll stay flat on his back, that should help you some of the way
Too true. I basically just need to be better at bjj, lolz
the "trick" is to keep your underhook tight with your own head between the arm you have underhooked and his head - this should put enough pressure on his to avoid him turning into you. From here on it's a question of moving your legs and hip independent from your upper body - which is easier said than done :-)