It occurs when the links between parts in the assembly have changed. The parts themselves may be up to date but the assembly needs to be opened to update the links.
Also occurs when mass properties of the assembly and sub-assemblies are out of date.
When I update parts I try to do so using edit in place from parent assemblies. On returning to the parent assembly I would then do CTRL+U to update all relationships, followed by a mass prop calc.
bc