... traders have attributed a big part of the move to what we first said more than a week ago would be forced buying by pension funds who were mandated to buy as much as $850BN in stock for quarter end to offset the underperformance of stocks vs bonds. 

To be sure, some doubt emerged whether that is indeed the case yesterday when P&I magazine reported that at least one California pension fund, the Los Angeles City Employees' Retirement System, had temporarily modified its asset allocation and rebalancing policies, which includes allowing the staff to defer rebalancing its asset allocation, a move which many if not all other pension funds are expected to adopt.

That said, it was not clear if the new LACERS policy would take place this quarter or in the future, and so traders kept buying on Monday, frontrunning what they expected was a last minute dash by pension funds to buy stocks.