The most important is that water is better at dissolving gases at lower temperatures. This means that the air inside the can has gone into the water, but the water cannot itself expand to fill the space because it's a liquid, leaving a partial vacuum inside of the can. As a consequence, the lid of the can is literally being sucked onto the top of the can: you have to apply enough force to overcome the suction in order to get the lid off.

This process is further compounded by the fact that the gas itself takes up less space at lower temperature, which further increases the strength of the partial vacuum inside of the can.