Partly, for me, it's exposure. The more I hunt and the more times I draw on animals, the less it gets me.
But excitement is excitement. The turkey that thundered off a gobble mere feet behind me, the bear that appeared out of nowhere to walk right in front of me, the deer that held tighter than a quail before pronking off -- those unexpected startles take a minute or two to wear off. I still get some shakes when I have just enough time to get the adrenaline dump but not enough to calm down. If it's fast, I hardly notice -- it's just autopilot. A short delay can get me all jittery. Just a little longer and I can calm myself down enough to lose the shakes again.