A contributing factor appears to be that large, rigid items that end up slightly over-filling the CTBs -- Andre was just opening up a CTB to illustrate the problem on the video stream.
This could be more of an issue for Dragon than with other vehicles since Dragon has many cargo stowage locations that have rigid sides, thus being intolerant to CTB expansion. Example attached.
Still, great to see cargo ops in Dragon - which is the purpose of all this great effort since 2006.