As a disc beats in it'll get more understable because your wearing the bottom off of the disc slowly. There is one course here in Dallas, veterans park, that is nasty for this because most of the landing area is dirt and rock. Ive seen noticeable change in discs from just a handfull of rounds. If you play somewhere that is mostly grass then you don't run into the problem. On the other hand I have my first disc that I ever bought, an old mold dx beast, that still flies relativley like new because I mostly used it for thumbers. It has tons of dents and small gouges but it hasn't gotten thinner because the bottom never wore down.
But that's just the explanation that I've come to believe. I could be totally wrong. And there is always the chance that a few really good tree hits will damage the disc in a way that alters the flight characteristics.