Sharing Violation

I copy a folder with files from one workstation to a shared drive, then from another computer I copy from the sgaed drive back to a the workstation, and on some files I get “sharing violation” and of course there can be none, all I am doing is moving files, and no one else is on the network. How can this be happening? The server is using active directory. If I try and copy the files a few minutes later it works fine. I would like to understand what is going on here!!!