As far as I can tell it's an unofficial hastag of self description to warhammer fans formed in the lack of an official social media presence from GW. I believe the #warmongers originated among horus heresy players, then spread to the 40k community in general.
Thanks Xenith, a lot of the posts (maybe just because of who I follow) seemed to have references to a few of the recent inclusivity* "spats". So I wasn't sure if it was also a solidarity kinda thing.
*Inclusivity good, prejudice and exclusion bad, can't see how people can argue that but oh well.