Custard usually goes like that when it's done too quickly at too high a temp. My MIL always does it when she makes custard because she always rushes it LOL. drives me mental ...

ETA - another thing when baking custard is to make your own little bain marie when cooking it - get a big backing dish, put about an inch of water in it (or enough to come half way up the side of the dish your custard is in) and then put in the oven.