The event that triggered the move was Universal's "ET, the Extraterrestrial," Spielberg's 1982 monster hit, which prompted Roy to ask board members, "Why didn't we make that movie?" When it was revealed that Ron Miller had taken a pitch on the script from Spielberg himself but had passed on the project, it was enough for the rest of the family to push Miller out.
Michael Eisner, from the new management team, was the force that pushed Disney to the derp side.
It was already derpish, just not so openly. If I can find the info I found before, I’ll post it. I tried searching to no avail but I’ll try again.