The fact of the matter is, in terms of swift initial selloff, strong rally off the low, and time until the retest of that low or worse (if indeed a retest or worse occurs), this stock market of the worst (and/or the deepest) recession in nearly a century isn't all that unusual. What is unusual of course is the extent of the rebound off the low.
While I can cite "stimulus" impulse, and a willingness to print and spend that knows no bounds (amazing the number of folks whom I know, and whose stuff I read, who seem to have no concept of "moral hazard" when it comes to what amounts to [to this point] the bailing out of hedge funds and private equity firms), in the end it's the phenomenon that explains all such occurrences since, frankly, the beginning of markets.
A phenomenon that Gustave Le Bon articulated so well in his 1895 classic absolute must-read book, The Crowd: A Study of the Popular Mind:
"In a crowd every sentiment and act is contagious, and contagious to such a degree that an individual readily sacrifices his personal interest to the collective interest."
"It is not only by his acts that the individual in a crowd differs essentially from himself. Even before he has entirely lost his independence, his ideas and feelings have undergone a transformation, and the transformation is so profound as to change the miser into a spendthrift, the sceptic into a believer..."