Intermittent "Select a Reference Library" pop-up on opening Word document

When I open some Word documents, EndNote starts up and prompts me to select a reference library. If I cancel out of it, it just pops up again a few seconds later. The only way to stop it is to choose a library.

My CWYW settings are definitely NOT set to open EndNote when opening Word, and this only happens every once in a while on certain documents. I currently have a document that does this, and 2 other people have had this problem with this exact document. 

Is there something connected to the doc itself that could be causing it? We are reluctant to send it to our client until we figure this out.