So, normal? No, I don't think it is. What it does seem like is possibly canine obsessive-compulsive disorder. You mention that he's a rescue from the streets and it's entirely possible that during his street time he reacted or moved into that position during defecation as a result of something happening on the streets and it stuck, it may have even happened more than once.
At any rate, there are many signs of OCD in dogs, and one of them is ritualistic behaviors (Manual of Clinical Behavioral Medicine for Cats and Dogs, Dr. Karen L. Overall, page 938) which this seems to fall into. There are behavioral and medicinal treatments for OCD if that is what he has, but you need to get a formal diagnosis and it should be interfering with his day to day life. Having said that, the behavior seems odd, but generally innocuous as you described it and so I don't know that I'd be worried unless it really is preventing him from do his business properly.
Now bear in mind, I'm not a vet nor do I play one on TV. This behavior seemed to have some fit with what I read on canine OCD, but this is not a diagnosis (sorry, but I feel I should make that clear).