Command line pdf conversion returning zip files

We use the command line converter to extract text from PDFs. This usually works well, but often it returns what appears to be garbage with a .txt extension, but which is actually a zip file. Change the extension to “zip” and you can open it and see, among other things, the content in XML. I just tried to export one of these files to HTML instead of TXT, and it worked as expected. Any idea what’s going on here? This is a regularly recurring issue.