Strip pit lakes are noted for blowing out. The spoil ridges are quite porous.
No compaction, just loose shale, clay, rock, and coal.

That so many of them hold water at all is a wonder. There are quite a few around here that
are just big swampy holes in the ground.

It's not unusual to have underground connections between adjacent pits. Years ago I
had access to a property that was "reclaimed" strip mine land. One year beavers plugged
the spill pipe on what we called the Big Pit. It over-topped and cut a spoil ridge at the other
end of the pond. The water level dropped about 12', and within a day or two the adjacent
pit, 300+ feet away by land, had dropped almost as much. The property owners didn't
attempt to find and fix the connection between the two pits. They fixed the blowout in the
Big Pit and installed a new spill pipe six feet or so higher than the level of the original drain.
When the Big Pit started to refill the Little Pit followed it. When the Big Pit filled to the level
of the spill pipe the Little Pit also stopped filling, but not before it had connected to another
small pit by over-topping a spoil ridge. They installed an additional spill pipe on that pit at
same elevation of the new spill pipe on the other pit. When it was all over the Big Pit had
become the Little Big Pit, and the Little Pit turned into the Big Little Pit.

It's quite possible that the hole opening up in Stressless Long Pond is what caused the Neighbor
Pond to blow out, rather than the opposite. I think Stressless is on the right track to fix his pond,
but I wouldn't want to lay odds on it staying fixed.