WARNING: This is neither news nor fact, this is 100% speculation on my part, I could be partially or wholly wrong in any of the following.
The timing of the Emergency Rule declaration leads me to believe that Musharraf has tired of the court’s dragging out of the challenges to his election as President of Pakistan. Come November 15th, he would be outside of constitutional authority if he did not either remove the uniform or resign as President. He wants to ensure that he has some position in directing Pakistan’s future, but there are only 12 days to go until he would hit the limbo of being uncertain of any position.
If he waits and they don’t make the decision and he hung up the uniform before the Supreme court decided, then he could be declared illegal as a candidate for president, and lose both the fallback of Chief of Army staff, and the future as President. I suspect he got tired of waiting on his long time political opponents, the Chaudry’s, to decide. It also makes me wonder if he had not given the court a deadline to decide by.