I just want to know how a tag gets listed in favorites even when I don't do anything to make a tag to be my favorite?
If you haven't added any favorite tags, the system infers some implicit favorite tags for you based on your activity.
See these questions for some details:
The tags that appear in your "Favorite Tags" list are added by you. It could be you added a tag in that list by mistake.
If you hover the mouse over a tag listed under a question, you will see a pop-up; if then you click on the star shown at the beginning, that tag is added to your favorite tags.
It isn't added to your favourite list automatically, you have to put it manually.
If you have some favourite tags you did not add, it's probably bug.