My Mother & I bake hundreds of cupcakes for our business a month from the same recipe, but the last few weeks half the cupcakes have been rising funny, keeping a flat top but splitting and bursting a bit at the side. (See picture) They were all on the back of the pan, but in our fan oven nothing else is coming out different. The baking powder is in date, and we mix them in a Kenwood maker so we're pretty sure it is well mixed. Any suggestions? It is not the end of the world as they get frosted after, but it is still really annoying!