Often, the speakers that are mounted in over-ear headphones are aimed toward the back of a person's head to simulate the effect of sitting in front of a pair of monitors. The ear cups may also be shaped so that they are only comfortable when worn with the right ear cup on the right ear and the left on the left.
Most music is recorded in stereo, which means it has separate recordings for right and left ears, and this fools our brain into thinking the sound is coming from different places, rather than a single source.