I am suspecting the beavers did the job. They like to raise water levels. A rental farm we farmed one time had a strip pit that was draining off into an old sinkhole from an old shallow coal mineshaft. I always thought it would be nice to plug that and keep the water from flowing out of the pit and into the shaft (only to create a sink hole somewhere else). But it was not my property so did nothing. Well a family of beavers eventually moved in, made a dam around the mine shaft sink hole that was at the edge of the pit, and raised the water level.
So if the pipe starts to open up from water flow after you got a small opening started, don't be surprised if the beavers "repair" your fix.
