Dave -
I too have noticed many a BD machine wanting to agitate a bit when in neutral. In fact I have several I could get to do that right now.
Inside the transmission, 'agitation' is always going on, whenever the motor is running. There is a gear fork which when activated by the wig wag and cam bar, pulls the agitator shaft 'out of gear'.
Through certain conditions, especially I think a minor thickening of the oil, OR a contamination thereof, parts inside don't slip as well as they did when new, and so the agitator shaft temporarily snags itself on the drive gear and it moves a little.
In a normal situation, wet laundry on an agitator in this condition should hold it down. As long as the agitator doesn't drag during spin, I would not concern myself about this whatsoever. Regular use of a machine may improve this also.