Not totally sure how that is happening to you but I'll ask the following follow-up questions just in case someone with knowledge on this subject can help you. But bear in mind that this forum is barely active and it can take several days for a few good replies.
1. Have you downloaded any mods outside of tf2? (Tools, skins, sounds, things that change your tf2 files?)
2. Do you use any third party tools (i.e. tools not authorized or designed by the creators of steam or valve) that make changes to tf2 or steam?
3. How long has this problem been happening? Have you done anything different since your problem first occurred?
4. Does anyone else use your computer, or have access to your files/steam account? You you suspect that they could be causing the issue?
Thats about it. Good luck.