Bubble Time - Perhaps if we look at history

An interesting chart on the history of rolling returns asks the question - Are return extremes associated with the business cycle or bubbles?  At the end or just after a recession, there usually is a nice jump in returns. This is a reversal of the past declines. This return behavior makes sense. Prices are driven down in a recession and risk averse investors need a higher return. 

The more interesting question is the driver behind high returns not associated with recessions in the business cycle. What causes these infrequent extreme?  There is the suggestion of a bubble. This term is thrown around, yet it may not have any substantive meaning.  Can high market returns just mean-revert? Returns post a 20% increase are likely to be lower, but that does not mean losses or a bubble.