some of the reasoning i've heard is a bit conspiracy theory-esque but friends close to the matter say that since Goldman are pretty much the only big traders left in NY (since Lehman and Bear went last year), they have always had a habit of proping up markets at quarter close if it's been bad or a reversal may be near since they can then sell more to their clients and make more money.
I don't think its a conspiracy theory, I think its an open secret ref: GS. The conspiracy is whether (or to what extent) the Obama administration has been involved with this recently - the taxpayer now counts among their client list.