Why do parts fall behind when moving them in an assembly?

[dpumaS63AX]

I've been working on a school project and it involves a pivot motion, however, when I move the parts they fall behind then when I stop jump to where it should be, I'm just curious as to whether or not I'm doing something wrong or its just a common thing. 

[JDMather]

@dpumaS63AX 

Can you Attach your assembly here for diagnosis?

Does Rebuild All on the Manage tab return any errors?

Are there any issues indicated in the expanded Relationships folder?