This Dirt Is Behaving Like Jello

I have no idea what the explanation is here, but the dirt at this construction site is behaving like Jello. Or behaving in some fluid-like ways - there is a spot later in the video where large waves ripple out. Very odd. There's a great explanation for this - it's not something that is all that uncommon, really, under the right circumstances. I just don't know what that explanation is. If you do, tell us in comments!