Water expands as it gets hotter.

"At 212°F, 14.7 psia, liquid water has a specific volume of 0.016716 ft3/lbm and steam has a specific volume of 26.80 ft3/lbm, which is a volume ratio of ~1600 : 1 of steam:water."

When racing, we had tires running as little as 15psi, and heating to as much as 230º on the center of the carcass. So it doesn't take much moisture inside the tire to cause the pressure to raise at higher temperatures.