One reason soros may be trying to defeat President Bush is because Bush's SEC department wants to regulate hedge funds. This would put a crimp in soros's shady and secretive style.
Its a little bit more complicated then that. Soros makes alot of money off of currency speculation and arbitrage (he's probably one of the best at it in the world, if not the best).
However, as if a bank making a bad loan, you want to recoup your losses, soros prefers activist treasury secretary's. A country that makes errors is prone to be run on by Soro's, but he needs to be able to invest and get his cash back if the country's financal structure falls apart.
If you remember some of the bailouts back in the 1990's that Clinton (really Rubin) engineered, the main benefactor was George Soros himself and also some major US banks.
Bush's administration is a bit to free market oriented then that and seems to follow more or less a market correction philosophy, i.e. no bailout, you screw up, this will teach you a lesson.
Soros is also fiercely anti-american and was opposed to the afghan war on the grounds that "thousands of people die every day, why is this so different".
Never the less, based on his financial strategy, and his methodologuy, its essential to have somone out there who can pay off the bills when nations get spend happy, hence why Soros support an international tax to support World Bank and IMF operations.