I don't think it was already dead. It looks as though he managed to get only so far when he got shocked then because he felt like he was being attacked he went to strike at the offender and got hung up by his teeth on the fence wire. Now with nothing else he started squirming around and really got wrapped up in the wire and was finally electrocuted.
Now if the electric shock was strong enough to kill immediately then he was probably electrocuted and nerves reacting made him coil around the bottom wires. I have seen this happen with snakes a lot of times even when decapitated.
Looking at the snakes scuff marks on the full size pix it looks like he was alive. There must have been something wrong with the fence to kill something that big. I think they are only supposed to give you a 12 volt jolt... I will remember this next time I go near an electric fence.