Speed wobble is pretty much the sum of forces on the bikes steering inputs that cause a harmonic oscillation of the front wheel. As published in science (and discussed a few pages back), we don't really know why bicycles will self steer, but we realize that they do. So if your speed/position/force of steering input is just right, your bike will self steer left, then right, then left, then right, and so on in rapid succession, causing the wobble.

To learn how to avoid it (it's not your bike, it's your technique. EDIT: well, your bikes geometry does have something to do with it, but 30mph should be very doable for most modern road geo) check out the post on speed wobble at cyclingtipsblog.com.