Varies depending on the site, sometimes it's gasoline, or solvents, or heavy metals or PFAS. As for how it happens, accidental or deliberate releases. I've found military documents from the 50s that say the official place to dispose of used motor oil was a pit they'd dug in the ground.

Here's a recent article about PFAS in drinking water. Very unfortunate.

Heavy metals and PCBs are most common in my area, various VOCs aren't far behind. Prior to the EPA and associated legislation companies would commonly use waste process waters for dust control, dump wastes in to pits or on the ground, spills would be left to soak away, and general processes were dirtier and uncontrolled.

One terrible example from western NY that bugs me even more than Love Canal is the involvement with the Manhattan Project. Local steel workers rolled Uranium and they were never told what is was, given any protections, or cared for when the inevitable happened. Radioactive waste was later used as fill for residential and commercial properties in the area. These Hotspot still exist and it is a slow process to get any cleanup done.