There are a couple potential culprits here. The most common one is that the mug wasn't placed in the center of the microwave. Microwave ovens have hot spots, and in most ovens, this is the center. When the mug is placed off-center, it will cook more on one side than the other, which will result in a lopsided cake. If your mug was placed in the center, it is possible the hot spots in your microwave aren't in the center, in which case you can try and move the mug around your microwave in future attempts to find the right place to put it.  Another possibility is that your cup wasn't oiled evenly. If there are areas of the mug that didn't receive an even coating of oil, the cake will stick to that area, resulting in uneven rising.  The final possibility is that your batter wasn't whisked together evenly.