There are stories of how the gods were going insane. Andruil, to be specific, going insane from the contact with the Veil. Mythal tricks her and removes the memory of the Veil's location from her mind and she goes back to "normal",more or less.
Falon'din also seemed to be insane, killing those who would not worship him/become his followers. It took the gods bloodying him in his own temple for him to stop his bloodshed.
My theory is the gods all attacked Mythal, their "mother" (they would really be the only ones with the ability to kill a god, I think). In an attempt to save the People and the gods, Fen'harel sealed them inside the Eluvian at the end.
I'm still, however, working out more pieces and hope to find more as I playthrough again.