This sometimes happens on a number of actions where files are burst-saved into the vault.
Never on copy/paste though.
I have seen this when saving files in SOLIDWORKS from an open STEP file, when unpacking a ZIP, and gnerating files from Treehouse, in addition to Copy Tree.
Would very much like to know why.
I have contacted my VAR about it and they sent log files to Solidworks. I will post an update when I hear something.