The main reason that George Bush is responsible for the financial crisis that his administration allowed the oversight of Wall Street to become far too lax. In particular this was the result of resistance that the administration had to imposing tougher regulation on hedge funds and mutual funds. In fact Bush's chosen director of the SEC would end up resigning because he didn't believe the administration was serious about imposing proper regulation. This is in large part what led to the financial crisis as many financial institutions made some very questionable decisions.
A big part of the reason that the Bush administration didn't really provide the kind of oversight that they should have is that they were elected in large part not to. They had run on a platform that the best thing for the economy was to reduce the amount of regulation that businesses had to deal with so that they would be more profitable. This combined with the Federal Reserve's policy of doing the same thing meant that the banks and other financial institutions were operating with basically on rules as to what they were allowed to do. This was something that they took full advantage of.
With virtually nobody enforcing regulation on the banks they were able to pursue policies which at the time where very profitable but at the same time very risky. It really should not have come as a surprise when this inevitably blew up in their faces. The main problem that the banks had was that they had purchased mortgage backed bonds that had all been bundled together. This worked fine as long as the mortgages were being paid, but as soon as interest rates went up and people had trouble paying their bills trouble set in. since nobody knew who was holding the bad debt it created a situation where everybody stopped granting credit.
The inability of people to pay their mortgages was another critical part in the cause of the financial crisis and this one was in large part the responsibility of the Bush administration as well. They had decided that one of their policy goals was to encourage home ownership in the belief that it would be good for the economy. The problem was this resulted in policies that encouraged people to buy houses that they couldn't afford. The policy that they pursued also encouraged the banks to provide mortgages to people who really couldn't afford them.
All of these people buying houses created a couple of problems, the obvious one was that many people couldn't pay their mortgages. The other issue was that it created a bubble in the housing market. Common sense would tell you that if the banks were granting mortgages to people who have wouldn't otherwise have qualified it was because they were running out of better qualified customers. That naturally meant that the number of people who were able to buy a house was declining which meant that prices had to decline. This decline in housing prices made the crisis much worse.
There are of course many other factors that contributed to the financial crisis and the Bush administration has to take responsibility for this as well. A major contributor was the massive budget deficits that the country was running. This was in large part the result of the two wars that the country was fighting. These deficits caused a few problems; first of all since the government was broke their ability to stimulate the economy through government spending was seriously limited.
The other big problem with the budget deficits that the government was running was that it meant that they had to borrow the money from other countries. Most of this money came from China which was putting money into the American economy in order to keep the value of its own currency down relative to the US dollar. This made Chinese manufacturers much more competitive and more importantly reduced the competitiveness of American companies. As a result most US companies sent their manufacturing overseas. This of course led to large numbers of people losing their jobs which hurt the economy in a pretty significant way.
Of course as all of these people started to lose their jobs it increased the number of people who were unable to make their mortgage payments which meant that more and more banks were getting stuck with bad mortgages. This made the problem even worse and it the economy went into a downward spiral.
Clearly George Bush is not totally responsible for the financial crisis, the executives on Wall Street need to take at least as much blame, as does the head of the Federal Reserve. However when all is said and done the cause of the financial crisis was the policy of the Bush administration to encourage economic growth. They created a situation where the economy was allowed to grow unchecked because it garnered a great deal of political support when things were going well. However it was just a matter of time before the whole thing collapsed which is what happened. This is something that they should have seen coming and taken steps to prevent. The fact that they didn't is why George Bush has to take a large part of the responsibility for the financial crisis.