Based on the MDT's long range scan, Hao Ren realised that beneath the vortex lay a hole that went far into the ground. This made it very possible for the vortex to be related to the missing spaceship. He suspected that the initial "crash" had already destroyed the geological structure of the lake bottom. The ship itself was just acting as a cork to seal the hole that it had created. This was probably why nothing happened until the Kerbalians went underwater to excavate the lost ship. That action of theirs had caused the weakened structure to start unravelling.
The gap did not cave in immediately, so there was probably a thin layer of crust supporting it. Hao Ren then remembered the void they left behind after the ship was teleported away. The subsequent crash of millions of gallons of water alongside the built-up pressure against the lake bed caused the whole place to just cave in.
