Whats going on at Thicket Excavations?

This was one of the first places I encountered after leaving the vault, and some guy there wanted my help fixing some broken pipes. I couldn't pass the persuasion check at the time so he wouldn't explain what his plan was. After a long time has passed, I finally went by there again and discovered a huge pit filled with raiders and legendary mirelurks, but the mirelurks weren't hostile to me.

What exactly was going on in this pit? I explored it and killed everything in it but wasn't able to find any terminals or notes of any kind explaining what was going on.

If you pass the persuasion test in the initial conversation, Sully simply remarks that he might want to use the quarry to dump bodies.

To quote the fallout wiki;

After the player character completes the Pull the Plug quest and leaves the quarry, upon returning after a while, they will find that it has become occupied by a large band of raiders. Descending to the bottom will lead to a confrontation with a masked, well-armored Sully Mathis, who may be difficult to defeat at lower levels. If the Sole Survivor found Sully's journal holotape, they can discern he was already a Raider.