I'm not exactly sure how to describe what's going on, since I'm too scared to go outside of my little hidey-hole to investigate. However, I think it's probably the piglins fleeing from the zombie piglins, but being prevented somehow by the soul sand beach that they're all on.

It's just a mess of ten or twenty iterations of the retreating noises, all stacked on top of each other.

Btw, I attached a short video of it happening, but I'm not sure that this system supports it.