If I remember if you are scrolling around in a tag and decide to make a post without leaving the tag, tumblr will assign a tag to your post because you were searching under that tag. I don't know if that helps, but it certainly is an option.

This sounds right…I bet that is it, exactly.

Okay, so if I am searching a saved tag for whatever reason, leave the tag before making a post.

I bet that explains it. THANK YOU!