The causes of 1990-91 and 2007-09 were similar: Real estate collapse, stock market turbulence, financial engineering turkeys come home to roost, loan failures, taxes going up, a Middle Eastern war, a spike in oil prices and everybody de-leveraging at once.
Another synchronous round of QE, for instance, may end up causing a trade war, while an additional debt-financed fiscal stimulus could cause a spike in interest rates.