Rigid body collision shape changes unexpectedly

Hello everyone!

I am working in Blender 2.70 with rigid bodys and different collision shapes, e.g. convex hull and mesh.
I am using the cell fracture and bullet constraint tool.

At some point during my workflow the collision shapes of ALL of my objects (both active and passive) in the .blend file get changed to the “BOX” collision shape. Usually I notice this quickly but its a tedious work, to change all the individual shapes back to their desired state. I can neither reproduce this effect nor can I identify, if I am accidentally hitting a hotkey for that to happen oder what the cause could be?

The objects are clearly separated into individual groups and belong also to the group “rigid body world”.

Since I could not solve the problem by reading the manual or browse this forum for similar problems, I would be thankful for any help!