HI jesse, I know now what is happening.
Mantaflow uses FLIP fluids. What you see is only the surface of the fluid. (The colors depict the speed of the water surface .)
because the water keeps flowing in the Domain, the Cube is filling up with water and the surface of the water rises. Also around the monkeys the water is treated as a surface. That is what you see. If I remember correctly, CGGeek turned of the ‘Use Inflow’ at frame 41 or so. That is why his Domain didn’t fill up completely and the water surface didn’t rise up so high;)
