I have a half of a theory

5000 karma yada yada...

With breaking the bounds of the game being much more prevelant in ch5, i have a theory on nature of dark worlds.

Its memory corruption, like in a programming terms.

Deltarune world is simulated one and opening dark fountain shifts memory table in such a way that objects are being interpreted as people. (If you are not fammiliar with the concept, go look up some Mario bros glitches or such, where sprites for enemies and enviroment switch places) Makes sense that opening multiple of them could crush the game.