I think Illustrator is leaving an IStream open.

I have hit this a couple of times. Once when running a tool that automates opening, exporting and closing a .ai file. But, also a couple of times when I started Illustrator and opened a .ai file directly and modified the file. Illustrator saved a  file with "[converted]" in the name. I think that was the string, close for sure. In any case, later when I went to move the file to another folder, Windows refused to let it copy. The reason is that a stream in the file was still being held by Illustrator. Closing Illustrator did not force the release of the stream. Each time it has happened, I have to restart the box to force all open handles on the file to be released.

Sorry I don't have more details. I was trying to change what turned out to be the artboard widths and heights. Took some trial and error. Also, at one point, and I don't recall if it was one of the files that I could not move, I fence selected all the geometry and grouped it. Moved it off the artboard, ran the artboard command and created a new artboard, deleted the original one. Then, renamed the new one so it was the same as the old one.

I suspect, as a programmer, that you have a reference leak on an IStream interface. But, not an easy one to find as this doesn't occur very often.