2 broken fan belts in 8 months?! '03 4.6is

[jcorreanyc]

So I was driving home tonight, it was 6 degrees outside in NYC, and a few blocks from the garage I heard some rattling from the engine. It almost sounded like plastic pieces flying around the engine so I got scared that it might be my timing chain guides that broke because of the bitter cold. I have 128k miles so you never know. I stopped the engine to make sure I wouldn't destroy it if that was the case. I popped the hood and realized that it was just the fan belt that broke and pieces of rubber got caught in the fan. I just replaced both belts 6 months ago after the fan belt broke last spring. AC was working perfectly (haven't used it in months though) and I even replaced both tensioners! What could cause two fan belts to break 8 months apart?