It makes a lot of sense to move forward at some point, but this time the timing is a bit suspicious: now that node.js started to have a lot of traction the lead developer stepping down is a bit unexpected. My guess is that there is some very exciting new adventure in the Ryan's future, or that sometimes three years are simply enough from a "feelings" point of view. Anyway best fishes for the future.

EDIT: edited a bit in the form to make my thought more clear.