Generations: The shockwave from the stardrive section explosion pushes the saucer into crashing into the planet. Same reason can be used from the massive explosion of mega-trops on the Vengeance.
One could argue that a warp core breach on a giant starship (huge matter-antimatter explosion) is orders or magnitude above several torpedoes detonating... and they'd probably be correct. The radiation pressure from the former would be much greater.
But in the end: Generations was shit writing. So there you go. I also hate Generations. Can you get off my back now?