Why do tennis balls bounce lower when frozen?

Answer 1

Freezing takes the material below its glass transition temperature, at which it stops acting like rubber and starts acting like, well, glass. Even if it's not actually below the glass transition temperature, it's certainly closer to it, and it also decreases the pressure inside.

Answer 2

The freezing-over adds more weight onto the ball, therefore gives it more gravity attraction, and bounces lower. Basically, all objects are like that.