[AXMonster]

Its not really much to do with Nitrogen, but more to do with water.

Air contains a small percentage of water vapour. When compressed this reforms into water and can enter the tyre when pumping it up.

As the tyre heats up the water vapour can evaporate and increase the pressure in the tyre as it takes up more room.

Nitrogen does not contain water vapour so does not cause as much pressure increase when hot. It also has larger molecules than the rest of the gasses in air, so does not leak out of the tyre as quickly.