It could be that your body can't keep up with the board. The board can hold down a lot of speed and power, your ankles need to bend and take shocks. The better the riding the more you want to push the board and you push the ankles too much.
Q: Could you ride the same way with a different strapped TT and not hurt your ankle? If no, then you could be having too much fun. Sorry.
Alternately, I ride the 150, I set it up to take some strain out. I moved the straps back to give myself maybe a half inch bias toward my dominant side - less leaning back over the tail. I moved the straps and pads toward the toe side of the board - the mako holds plenty of heel side naturally so you can give a little up to dig in better toeside. I also ride without the front fin which doesn't have as much to do with comfort as sweepiness. I'm a little compromised on my weak tack but that's not where I'm playing anyway.
It's never going to give you the freedom of strapless though.