Help Please - There are bubbles under my crust
I've been making the 100% Whole Wheat Bread recipe on the back of the King Arthur Whole Wheat Flour bag for about 3 months now on a weekly basis. The last few times I've made the bread, I've had an issue with air pockets under the top crust (see photo below which is about the worst I've seen yet). I haven't changed anything regarding the recipe and did not have this issue the first couple of times I made this particular recipe. I thought I might be overproofing (generally I do about a 50 minute first rise, and a 30 to 40 minute rise in the pans after shaping), but I'm not sure if that's the case. Any help anybody can give would be greatly appreciated. Thanks!
-Darron from The Teacher Learns to Cook 
Wheat Bread Bubble Under Top Crust