You think Trump might have submitted his own tax form anonymously just to make people think it was a huge tip when its was really nothing and hope they get up in arms about it? Sort of like troll baiting?
I wouldn't rule it out. It was a client copy, after all.
I suspect that at least some people who are currently demanding the tax returns will give it up, even though this particular document really doesn't advance the discussion in any meaningful way. To me, it looks like there's no way the leak could hurt Trump, and there's at least a small chance that it will help him out. So, to me, it's reasonable to think that he leaked it himself.