Well, the Dow did not return to its peak close in 1929 for 25 years.Do you mind me asking why 10 years? The longest bear market was about 3 years in length.
And Japan's Nikkei stock index has still not returned to its peak close in 1989 - that's 33 years and counting.
But to answer your question - there is no particular magic about having 10 years of anticipated expenses in "safe" assets at retirement.
It just seems like a reasonable balance to me.