Exactly what did-in the Soviet Venera spacecraft that landed on Venus?

The only two significant issues are temperature and pressure.

The surface pressure is over 90 times the atmospheric pressure at the surface of the earth. The surface temperature is over 730 Kelvins thanks to the greenhouse effect. I do not know the exact reason, but it seems like a combination of the heat frying the electronics and weakening the metal in the spacecraft so that it failed structurally, probably did the trick. You can survive the high pressure for a while, but electronic equipment does not like to work at these high temperatures. My guess is that the temperature was more important in causing the spacecraft to fail.