strange behavior

[mazzama06]

I have a sheltie that is a rescue (4years old neutered male). He has all sorts of issues, but he has this behavior I am not sure what he is doing. He actually doesn't do it a lot,(about once every few months) and he usually does it when I am taking him out to go to the bathroom. As I stand there waiting for him to go, he will suddenly just jump with all his weight and force at me. Now this dog is a very gentle dog when he jumps up to greet someone, and he really isn't a playful dog. So this isn't your typical jump at you to play behavior. He can sometimes do it more than once. He also gets a very wierd expression on his face, and I am just not sure what he is doing. He doesn't bite me when he does this, but he does push hard at me. I am guessing this is not a good behavior, but I just have never had a dog do this before, and wondered just what he is doing.
Thanks,
Marie