Surface tension. If a liquid is spilt from a container too slowly, its surface tension will cause it to stick to the surface of the container, causing it to dribble all over the fucking place.

But, if you cause the liquid to spill out quickly, it greatly reduces the attraction of the liquid to the container, causing a much more fluid (no pun intended) spill.

To put it in an illustration, imagine you have a set of monkey bars over a trapdoor. Your job is to stand over the trap door and grab ahold of the monkey bars so that you don't fall through the trapdoor when it opens up. Not so hard to do, right?

Well, let's up the ante. This time, you need to stand on a platform that's a good 15 feet above the monkey bars. Now, when the trap door opens, you need to jump off of your platform and grab ahold of the monkey bars.

The velocity at which you're traveling by the time you reach the monkey bars will effectively make it more difficult to grab ahold of the monkey bars because of G-forces.

I realize that's probably a godawful illustration, but it's effectively the same concept. Surface tension allows liquid to "grab ahold" of a surface, but when velocity comes into play, it makes it more difficult for the liquid to stick to a surface.