The antenna converts electro-magnetic energy into electrical energy, which is basically electrons moving into your radio.

There are only so many electrons in each inch of copper wire, so when they've been sent downstream into your radio, the wire will become "ionized" and deteriorate and probably fall down. This explains why, when you come home one day, your antenna is on the ground (see below).

What happens to all those electrons, you ask. Well, they migrate into your radio and accumulate. In older tube radios, there was a "grid leak" resistor circuit which allowed the electrons to fall on the ground. Now you can't see them, but they're there. As more pile up, they slide into your back yard.

Tube radios, because of the "grid leak" last a lot longer than solid state radios, which stop working when enough electrons have piled up inside to short it out.

Now those electrons in your back yard want to get back into the copper wire, so they "pull" the antenna down to be re-united with it. Since the antenna is high, and they're on the ground, this attraction is not strong, but on a windy day, the electrons get lifted from the ground towards the antenna, pulling it down again. The wind oftens brings in free electrons from your neighbor's homes (from TVs, etc), so there may be a lot of these things around. If too many electrons get lifted up all at once, they overload the antenna, causing a heat mark, or worse getting back into the radio. Now this is why your antenna usually falls down on windy days.

You can extend the life of your antenna by disconnecting it from your radio when you're not listening. But overall, 500 to 1000 hours spent listening will do in a longwire antenna.