The LibreOffice installer provides an option to associate it with various MS Office file types. By default, Word documents are not selected, but Excel and Powerpoint are. I unchecked all the boxes, but after installation LibreOffice is associated with... all MS Office file types including Word documents.
Reproducible with Version: 5.4.1.2 (x64) Build ID: ea7cb86e6eeb2bf3a5af73a8f7777ac570321527 CPU threads: 1; OS: Windows 6.1; UI render: default; Locale: en-US (en_US); Calc: group
Well, on the second thought, it might not be a bug. This only happens if the files don't have default applications assigned. LibreOffice doesn't actually assign itself as default application, it registers itself as an application *able* to open these files. E.g., it writes itself to the HKEY_CLASSES_ROOT\<.EXT>\OpenWithProgIDs, and to HKEY_LOCAL_MACHINE\SOFTWARE\RegisteredApplications, which points to Capabilities. It appears that Windows itself selects the only available application able to handle the file extension as its assigned handler. This may be seen with .docx, which has default handler out of the box (to WORDPAD). Installation of LibreOffice does not change this, it only adds a new item to Open With context menu. Since it's an OS feature, not something that LO installer does wrong, I suppose it may be closed NOTABUG.