One of the major hypothesis is that Venus was hit by another planet at an odd angle and it's rotation was reversed, if that happened it might have set up a hot house simply by vaporizing the planetary crust. this would have resulted a hot house from the very start that the slow rotation of the planet just amplified.
Collisions are thought to have been frequent and the angle and directs of collisions were important, Mercury would appear to have been hit so hard it threw off most of it's crust. Venus probably merged with one of it's late impactors and the Earth was hit at an angle that caused much of the crust to be splashed out into space to eventually become our moon.