I'm don't mean to suggest trees drop any sort of percentage of leaves. More the fact they do shed leaves (just like Startups shedding staff) in times of water stress.
"... Or... just before they go dormant for the winter... "
Maybe from where you come from. Not where I come from, a land of non-deciduous trees. There is a big difference between deciduous leaves dropping in Autumn and trees under stress. Check the deciduous Oak here loosing leaves in Autumn (yes I keep a visual diary of 1 particular Oak through the seasons) ~ http://www.flickr.com/photos/bootload/sets/72157607419227026...
Then compare this to a non-deciduous Eucalypt in stress ~ http://www.flickr.com/photos/bootload/493280400/in/set-72157... (yes I also keep a visual diaries of dead trees) ~ http://www.flickr.com/photos/bootload/sets/72157607633560765...
It's called die-back. We get a lot of it where the leaves are shed from the farthest limbs in order to help the core survive. The metaphor I use in this situation is apt.
Think this will be the last round of layoffs?
Probably not. A salient point to remember about creating/joining companies with VC backing they become another layer of management dictating the terms when financial conditions are not in their favour. As for shedding the bottom 5% - trees do the same. Shed their leaves in a desperate bid to survive just before they die.