Good question. It is a little difficult to know for certain, but generally speaking if you are confident on your bulk ferment then it is mostly likely your proofing. I recently completed several rounds of testing with the Purist. I found myself up late, 11:30 pm, 3 hours into the proof and I knew they needed more time but I was tired! So I went as long as I could, meaning as long as I could keep myself awake, then popped them in the oven. Of course, I got a little spliting on the sides. They were in the zone. But if I had given them another 30 minutes I bet the splitting would be non-existent!. Test this out yourself. If you have a multiple loaf batch. Bake one “early” or when you normally would and then bake the next one 20 to 30 minutes later. It is a great science experiment in your kitchen.