I had a bit of a restless night, one of those ones where your mind just doesn't switch off and goes through any number of random subjects. However, one thing during this mild delirium phase related to Doctor Who popped into my head which appears to me to make sense...
Rose being back from the alternate universe is down to the last Dalek, who has recreated the link in order to pull the alternate Skaro through to our universe. In the alternate Skaro, the Kaleds and Thals never fought their war which resulted in the creation of the Daleks, however the wheelchair-bound Davros is still their lead scientist. When the last Dalek meets Davros, he is convinced again to mutate the Kaleds to create a new legion of genetically pure Daleks, which in the absence of the Time Lords will rule both realities.
In order to pull the alternate Skaro through the void, the mass between the universes must be altered to make the alternate one mass-heavy, therefore the last Dalek is destroying planets in our universe and pushing the resulting matter through the link to create the right conditions. And because the link itself is located between our Earth and the alternate one, getting it to the right size to bring a planet through will destroy both Earths in the process. Although doing so is changing history on the alternate Earth, possibly even getting rid of Pete again (hence Rose looking so miserable), Rose, Jackie and Mickey are completely aware of the changes as they occur because they aren't from that reality and have come to gain help in stopping it.
Phew. Sound plausible?