It did that during a scheduled scan - I believe after I updated Wordfence(the 4.0.2 update).
I installed it from the plugins repository and I did so quite some time ago. The trac log shows that the last commit for the Events Manager plugin was 6 weeks ago, so I'm not sure why it would show up just now.
https://plugins.trac.wordpress.org/browser/events-manager/ .
Basically there's been no change whatsoever in the site where I experienced this happening(besides updating to Wordfence 4.0.2 - I also just updated to 4.0.3).
So I assume that when you download the plugin now, you would actually have the "= 5.5.2.1 (dev) =" section in the readme(since it's in the tagged directory as well).