There are two reasons for bicycle pumps becoming hot during usage. The first reason is the friction that generates through the motion of the piston and cylinder’s wall. And the second reason is the increasing contact between air molecules. If the pumps get too hot, it’s time to give it a break and let it rest for some time. Otherwise, it may explode or do some damage to your tire and the pump. These are minor details about the bike and you better research such situations before buying a bike. Pre-planning your purchase is better than learning a lesson after the damage is being done.