Why did Proto-Germanic break up?

As far as I know, most agree that Proto-Germanic broke up into West,
East and North Germanic around the beginning of our era (between 1
BCE and 1 CE). Of course I have my own scenario of how that happened,
but I would like know what in the "standard version" is the cause or
occasion why this should happen (migration? war? communication
breakdown? common agreement that this would be a good idea?) after
such a long time of common develæopment?

Torsten