Bush mongered a war with no way of paying for it. He returned what could have been a budget surplus in rebates.
He did nothing to put in laws that would have prevented the real estate meltdown.
That was never in the cards.
First thing that happened was the Baby boom started retireing , soon the ratio of people paying into SS was no longer suficient to hold up the whole budget.
Social Security is in no respect diffrent than a Ponzi scheme, it is just a large one with a lot of inertia, during the Clinton years it peaked.
There may be any number of other factors , but this one by itself is so large that all other factors are swamped.
Check this out, get the figures for the Clinton surplusses that span about six years, then for each year subtract the Social Security component, Voilla, "surplus" entirely disapears.
Early in the Bush administration this started happening as the Baby boom started retireing in great numbers.