Another possible trigger for updating during loading is positioning constraints. Similar to the suggestion from KhimaniMohiki, delaying these position updates may help. As Cowski suggests if there are interpart links in the assembly defer those as well. (these links may have occurred implicitly for various reasons (assembly cut, deform component, routing port selection, selection outside work part)
As multicaduser and looslib point out the parts may not have saved to the current NX version. This would also occur if the component parts are part family members. Open the part family template and update the members using the latest NX version. Use info, part, loaded parts to check the NX version of the parts.
The NX status line may provide insights in to what and why the updates are occurring. Are there out of date sheet metal flat patterns in any of the parts?
HTH, Joe