I think the theory goes that by intervening we essentially created the first wave of zombie companies by papering over losses with printed money. The effect was to smooth over the immediate pain but prolong the downturn. There are some that argue that if left alone, the market would have violently corrected in short order as it had on previous cycles. By not allowing it to do so we have created an insurmountable debt load.
I think thats the general theory.