One reason why upturns follow downturns is that downturns tend to overshoot. People get panicky, they're afraid to stay the course, so they start selling. The other thing is that I think, as entrepreneurs keep on waiting to produce new things, that there's an accumulation of as-yet-unexploited new ideas that keeps mounting up.
Entrepreneurs have only the murkiest picture of the future in which they are making their bets, and also there is ambiguity: they don't know when they push this lever or that lever that the outcome is going to be what they think it is going to be - there is the law of unanticipated consequences.