My own read is that the pile on to date has largely been confined to state legislators and mayors and Democratic partisans in general. Booker clearly wants to make clear he's not part of either club. Remember, up until very recently, Christie had a get-along and go-along relationship with many state Democrats.
I think Booker wants to stay on the sidelines (a standing trait) until it's clear that Christie is really sunk. Let's call it the Howard Baker model. Then he'll come in with "more sorry than anger" as they say and let the hammer drop.