Well, one reason why upturns follow downturns is that downturns tend to overshoot.
There is a concept in financial maelstroms known as the overshoot.
mid-14c., "to shoot, run, or pass beyond (a point or limit)," over- + shoot (v.). Related: Overshot; overshooting.
overshoot o·ver·shoot (ō'vər-shōōt')n. A change from steady state in response to a sudden change in some factor, as in electric potential or polarity when a cell or tissue is stimulated.