Drag one sub-assy, another moves, and changes colour.

When dragging a sub-assembly another sub-assembly moves, one that you wouldn't expect to.  The two sub-assemblies and any connecting sub-assemblies all simultaneously change to the same colour, usually red, but recently green.  

If anyone can explain why, I would be most grateful.

Using SW 2009 SP 5.0

SolidworksAssemblies