By heating up the drum and sealing it you have increased the temperature of the air (gas) inside.  According to Charles law, if we increase the temperature, we increase the volume of the gas.  But by putting the cap on the drum the gas can’t expand anymore and the pressure of the gas inside increases.  When the drum is dropped in ice water the temperature of the air inside decreases and so does the air pressure inside. The air pressure outside the drum is so much greater than the air pressure inside (because of the decrease in volume) that it crushes the drum.

Crush the Can

The molecular activity inside the drum is visualized here in this interactive animation. Click on the “Continue” button to start.

This animation simulates the rapid movement and escape of molecules as the “can” heats up and the gas volume increases.  When the lid is placed on the “can,” the molecules can no longer escape as the volume increases.  Hit “continue” again and you will see that as the can cools, the volume inside decreases and the “can” collapses.