Dirt in power steering resivoir

Two or three weeks back I went for a good 'ol mud bog as the snow was melting, and as I was coming home my power steering went out. I let the vehicle sit for an hour and then jacked the truck up steering left/right about 20 times to get all the air out and this seemed to work. Fluid looked ok, levels were fine, and no leaks in the lines or rack, everything worked as normal.

Fast forward a week, and the power steering pump is whining its ass off all of a sudden, I check the fluid and there's mud/debris in there. I'm going to give it a good flush tomorrow, and failing that I'll rebuild the pump and go from there.

But my question is, what the hell happened? How did the dirt get in while no fluid escaped?