Why did the pipe explode in The Shawshank Redemption?

I've done some more digging on this according to moviequotesandmore

When the top of the pipe is broken open, it shoots a fountain, indicating that it is pressurized (if only by gravity). But then the pipe drains nearly empty. It should only have drained to the lowest point in the hole broken at the top.

and there's an even more detailed discussion on Slipups That discusses such things as fluid dynamics, the angle of the pipe and the effectiveness of the shot i.e. getting the audience to feel revolted for what Andy had to go through.

I think that this was a deliberate mistake that the director decided would enhance the shot.