The edges of the cake get hot faster than the center. As a result, the cake batter around the outside of the pan sets first, stopping it from rising any further. The batter in the center keeps rising, forming a dome in the middle of the cake before it, too, sets.
To prevent or minimize this effect, you can buy strips that wrap around the outside of cake pans, slowing down the heating/setting process.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan