Some forums said this could be because of air incorporated, but I was very careful about this my second batch and it didn't make a difference. Another blog said to take your canelés out of the oven if they start to rise and poke holes in it. This reduced the overall temperature of the canelé and the oven from opening and closing it so the inside texture was off. Yet another blog said to just take them out of the mold once fully baked and let them deflate on a cooling rack. This kind of worked, but left them lopsided. Highly recommend avoiding these. I baked a few batches at 350°F and 400°F and kept having this issue despite starting the bake at 550°F to form the outer shell. I also realized the left side of the oven had a fan blowing, causing a few canelés on that side to mushroom while the others were steady. Once I cranked up the heat to 450°F for the last 30 minutes, I didn't see ballooning but instead a steady rise that eventually deflated.