For specific rules, you can look at the Steam Workshop terms of service - but as a very basic guide:

• Tracks should either be created by you, or you should have have specific permission from the creator to upload their tracks to the Steam Workshop.
  
• The tracks should be "family-friendly" and not contain any obscene content.
  
• The tracks should not contain anything illegal, or promotion of anything illegal.

Looking at the tracks you submitted, it seems they were not your own tracks and, as such, were removed.

Hope that helps!