In Rise of the Cybermen the TARDIS accidentally falls through the void and lands in a parallel universe. I've never been clear on what exactly it was that caused this to happen and, more importantly, why it would be unlikely to happen again. Given the finality of the Doctor and Rose's farewell, it is implied that it is impossible to ever return to that universe.
I have a few hypotheses: Perhaps it has something to do with the spacetime rift in Cardiff or possibly that it was due to the weakening of the walls between the universes caused by Torchwood's experiments.
Is this ever explained in the series?