the female bartender rubbed my back as she walked by. Why would she do this? we never talk about anything, I just order food and drink beer. She doesn't seem to be interested in me, though she is always busy Because she is at work. I have been to this bar alone quite a few times now, but have never spoken to each other, just ordering food. So why would she rub my back?
She sees you a regular and sometimes with regulars, whether you talk or not, you sort of obtain a comfortable familiarity, learn to like them. It's very possible it was a way of saying "hi", letting you know she noticed you.
Women, in my opinion, use flirty eye contact and facial expressions first before any touching to show interest. Women are usually affectionate like that in a platonic sense, and it could be if you are a regular customer she is showing a friendly appreciation or may have thought you looked sad or worried at the time.