I feel like I'm not trying to bump him when I'm posting, I think I just don't have good control of my lower legs at the rising trot. I don't seem to do it when I sit the trot. I should probably post without stirrups more, but he takes advantage of me not having stirrups and tends to dive into the middle of the ring or spin.
Looking again, I think part of what's happening is that you're riding off the back of your lower leg instead of the backs of your thighs and the inside of your calves, in trot. This happens because of a chain of events:
Tight hips + rolling forward onto the pubic bone = knees and toes turn out
With the toes turned out, the strongest part of the leg -- and the part most available to you to use -- is the back of the lower leg.
With the pelvis rolling forward, your abs aren't engaged and aren't helping you the way they should in rising, and the back of your thighs aren't as available to you either.
So instead of getting the lift from your abs and the backs (not the insides, but the backs!) of your thighs, your using the back of your lower legs to squeeze in and thrust you up for the rise.
You can fix a LOT of this with no stirrups work. If you're worried about him spinning or losing his steering... just don't steer him. Let him go wherever as long as he's trotting. Get a grab strap for your saddle, or grab the pommel. One-hand the reins, and with the hand that's hanging on to the strap or pommel, work on pulling yourself into the saddle while keeping the front of your pelvis lifted, your thighs down and back, and everything below the knee completely loose and floppy. Carry a whip so you don't have to keep asking with your (relaxed!!!) lower leg.
(I've been through ALL of this and it's something I have to check in on with myself a lot!! My hips tighten and I start to slide back into fork seat if I don't do this regularly.)