I say usually the rider, but it can also be the gear too. I used to get them all the time, but to help I would tighten the trucks, and ride, and after a while start to loosen them bit by bit as I got comfortable.
Usually its just a matter of practicing and building up your ankles and legs. After a while, your body because comfort with the board and just reacts naturally to the sight changes in your board when you're going DH to the point where it's just an involuntary reaction. Being nervous and thinking if you're going to crash doesn't help. Just take it slow till you build up the confidence and experience.
As for gear, some gear does help, but it's mostly experience and preference. I have a set up reys. My buddy of mine has a set of bears with a regular pintail. As much as I hate to admit it, he's DH game is far better than mine an can out ride me in that area.
@Agek I'm not saying you need brolic leg muscles or anything, but your muscles do need to build up over time. Holding a tuck for long periods of time can tire your leg pretty quickly if you don't use those muscles. Same thing with your ankles. They help with keeping balance at high speeds. I sprained my ankle a few weeks back. I'm a pretty decent rider, I could hit about 45ish mph, but at this present moment, I could barely hit 25 without woobles. I haven't change my gear at all, but my left ankle which is the one I ride with isn't up to its full strength. Most people don't think of it, but building up your leg muscles is pretty important. They naturally build up over time as you continue riding. It's just something that happens without you even noticing.