It's part of a JOSM workflow I use when editing *.osm files manually. JOSM will only upload objects it thinks has changed, and one was to mark the objects as changed is to make a change in JOSM. So I add "rjw:update" tags to the objects I want to edit, save the file as *.osm, manually edit it, reload it in JOSM, remove the "rjw:update" tags and then upload to OSM. I forgot to remove the tags this time.

Thanks for spotting it; I've now removed them all.