Here's my thought, if anyone has any evidence for this please share. This may also have been discussed before.
Let me pose two scenarios. The first scenario is one where you retire in 2009 when S&P 500 is at its lowest. Your stash is invested in 100% stocks (for this imaginary scenario). Your stash is also 1 million dollars. Thus you are withdrawing ~4% or 40K a year. In 2009 when the markets were tanking you own 1464 shares of S&P 500 which cost $683 a share at the low. (I know we cannot time the markets)
The second scenario is quitting now. You also have 1 million dollar stash. Your buying power with the million is only 473 shares of the S&P 500 at $2011 a share.
Since we all believe the markets will always go up since society as a whole will continue to innovate, consume, and use. It seems as if retiring when markets are going through the roof would be ill advised, even though our stashes seem to be at an all time high. Thus we would be withdrawing 4% of a stash which was built with all these up years.
If you had the choice wouldn't you want to retire when your stash is artificially deflated aka 2009, vs now when it may be inflated.