As I understand it, JRWorker is a process that is used during file imports. I think it reads tags and other data from audio (and/or video) files.
If you haven't been using MC for very long, MC may be importing all of your files using the Auto Import configuration. Or, if you've added a bunch of files, again, auto import may be processing through all of the new files.
It could also be rebuilding thumbnails, or analyzing audio. I would advise leaving it alone for a few hours; it should stop once MC is done processing files.
I'm not sure what else might trigger this behavior.
Brian.