Whats going on with that back leg?

Definitely draining. Could be a puncture wound that is infected or some necrosis happening. To me, it looks like the staining is larger than the actual wound which means it either punctured deep or hasn't healed right

shot out of season?

Very possibly a snake bite

His leg is swollen all the way down pretty good.

Looks like he has a tear on the rear of the left ham up high, and the right flank in front of his hip.

bullet wound

I have seen horsefly bites on the shoulders and hindquarters bleed a time or two. Those bald spots on the HQ and shoulder area can be from biting flies landing in those spots and the deer licking at those bite spots to shoo the flies off them. Those bald spots will usually be raw or bloody red when the flies get really bad. I have bucks out of velvet here as early as 10 days before the end of August so he could have been poked in the butt by another buck that was in hard horns already if the date is correct.

Barbed-wire fence maybe? It looks like there is a wound on the front and back side of that right rear leg.

This. Looks like he sat down on a snake.

Could easily be a fence wound.

Here is a trail cam photo of a doe that has bloody fly bite areas like I was mentioning above.

I don't think it is swollen. It is just camera angle that is making it look that way.

You may be right
Would like to see another picture or two from different angles.