Midori and Google Music Frame use WebKitGtk. LibreOffice Mozilla plugin causes LibreOffice is opened when WebkitGtk initializes plugins.
See https://bugs.launchpad.net/midori/+bug/824770 and https://bugs.launchpad.net/nuvola-player/+bug/890712
An unusual, Ubuntu bug - would be nice to confirm though.
Having said that - I have little sympathy for the idea it's a LibreOffice bug - if the plugin works correctly in Mozilla (which it appears to), it seems reasonably clear to me that WebKitGtk must be doing something different to Mozilla as it re-uses these, to get it to launch on startup - and hence this is a webkit bug.
We'd need to get whomever wrote the webkit / mozilla-plugin bridge to look into it; so - tentatively marking notourbug: if you can get more, detailed information from those guys on how to improve our plugin please do re-open.