I strongly suspect you somehow set it accidentally manually. This isn't something that I think can get set automatically by anything, though if so, it might be in a CUE sheet somehow. If it comes back, then we can investigate that. For now, assume you did it by accident when meddling places you shouldn't have been meddling.
It is super-simple to clear, as MrC explained above, but I'll try again. Do this:
1. Go to any View in MC where you can see all of your Audio Files.
2. Control-A to select them all.
3. In the Tag Action Window, find the Playback Range tag.
4. If it says "Varies", select it, and delete the contents.
5. Hit Enter. MC will complain that it is going to apply the tagging change to a crapload of files. Say OK.
Drink a beer. You're done.
The Playback Range tag is a feature. It allows you to specify that for a particular track, you only want it to play from timecode X to timecode Y. If you did not intentionally set this kind of restriction on any of your files, you can remove it globally without issue.