I'm not so sure that this is always a cranky "you can't make me" thing.
My Standardbred always does this big leap into the air to get into a trot, the first few times I thought he was about to take off or start bucking. But then he trots on quite happily.
He does it every time, on the lunge as well and even in his paddock when he wants to trot of his own accord!
He raced as a pacer not a trotter, its like he's having trouble working out which gait to use or trying to get his balance.
Reading what tinyliny wrote...
"If he is chucking his head down, while his front comes up, he is using the weight of his head to lift his shoulders. This is not a sign of pushing from the hind end, but just the opposite; it is of a horse that pulls more with his shoulders. This is his push off, but he needs the leverage of his head going down in order to lift his shoulders." Im going to watch closely next time to see if that's what he 's doing..
I'm would suspect its something to do with racing in harness for years..for my boy anyway.
Not sure how you teach them to push from the hind though...oh well, another thing to work on!