I ran into the same issue yesterday. Couldn't figure it out at all. I had 4 sub assemblies that I moved from a different higher level assembly and after they moved they exhibited the same charactersics you described. I managed to fix mine by right clicking in the design tree and selecting "Reload". I did it on one of the problem assemblies and all were fixed.
Hope this helps you.
Unfortunetly that method did not work for me.
I have yet to find a set of conditions that cause this to occur either. It seems entirely random from my end.
That being said, I have since upgrade to 2013 SP0 and have not had this occur again since. BUT being random and all, I have no way to test if this problem still exists other then continuing to work and see if it shows it's face again.
I'd try upgrading to 2013 or even 2012 - SP5.0.
If I don't have this problem occur again within the next while, I'll mark this as 'Answered' and chalk it up to a bug in 2012.
I too am having this problem, but I am running SW 2014 SP4. Any help would be great!!
Run the SolidWorks Rx > Diagnostics tool to see if it reports an error with the graphics card or driver, or anything else.
Most (if not all) of the problems listed above are graphics related. The card may be failing or the driver is corrupt.
It cold also be mouse/controller related.