I suppose you have to be sure of the rockets detonation point to know if you are moving away in the right direction. I'm not sure that is a thing you could be sure of the moment the rocket is fired (or if you would know).

Also the target (at least initially) was large formations of Tupolev Tu-4s who can only climb 15m/s. So that puts them right on the edge, but assuming more than one rocket is fired or that they simply don't always have time to realize it was fired quickly enough... that probably does it.

Why 150m? It says 'radius' so if the missile is on an intercept course, 150m is not going to help you. Beside the difficulty of knowing the exact launch time and having to move a 50mx50m 180 ton bomber in fewer than 12 seconds.

Fighters and interceptors can, bombers can't.

300m seems like a very conservative estimate for an airburst nuke.