Microwave ovens use radio waves to heat food. Quite a lot of them, around 1000 to 1500 watts, all of that trapped inside a metal box. Contrast that to a powerful radio station at 50,000 W that sends it off into space, so by the time it gets to you its microwatts.
Antennas work because an electrical field "induces" an electric current in a conductor. Antennas are made of conductors, typically aluminum (sometimes copper or steel). Radio waves are, in effect, a combination of electrical and magnetic fields. When the electrical part goes by the antenna it induces a current in it. You pick up that current in a wire and amplify the freak out of it.
Now what might happen if you put a TV antenna, which is used to getting microwatts of energy, inside a microwave oven where there's thousands of watts? Maybe you'll get a WHOLE FREAKLOAD of current?
And what happens to the temperature of conductors when they carry a WHOLE FREAKLOAD of current?
And what did you put in the microwave? Aluminum you say?