Why would a 63 amp fuse holder get hot?

The only time that fuse holders gets hot is when there is a loose connection between the fuse holder and the wire connected to the holder. If this is happening remove the supply voltage from the circuit that the fuse holder is in. Remove the wire from the holder. Cut the burned end of if there is enough wire and re install. If not enough wire, use a wire brush to get the oxidation off of the wire. Apply a coating of anti oxidation compound on the wire end and then re install into the holder. Tighten the wire into the holder very tightly. Check the spring clamping pressure of the fuse end of where the heating occurred. This spring clamping pressure is important for low resistance conductivity. If the fuse feels loose when installed change the fuse holder for a new one.