Probably the oven is too hot and the sides cook too quickly so that the middle of the mix then expands and breaks thru.There are other reasons - but that would be the most likely. Try dropping the temp about 10-20 deg next time.
Hi, I find that when I am baking I DONOT use the fan. Doing this gives a much better result. Hope this helps,Trudy
neither does mine judy lolILB i'm sure i've read that a very damp (but not dripping) towel / tea towel wrapped around the outside of your tin helps. but i tried to find the source and couldn't.