Here's my explaination:

Pressure is a physical quality of contained matter, in this case air inside a tube. Pressure is defined as the force per unit area exerted by the contained matter on the container walls (SI unit of pressure is the Pascal, which is equal to one Newton per Square Meter). When you have a gas in some container, the pressure results from collisions of the gas molecules against the walls of the container. As the temperature goes up, the kinetic energy of the molecules increases and there are a greater number of collisions per unit time. More collisions means more pressure. If you had already filled the tubes to capacity and then they were heated up (in a hot garage, for example), the increase in pressure might be great enough to pop the tubes.

Ain't science fun?