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.
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.