I noticed over the last week that my zFTP and zMAP changed suddenly, with my FTP going down and zMAP going up significantly. As I am racing ZRL, this put me on the cusp of promotion to a higher cat, and it affected multiple riders on our team.
Has anyone else noticed the same? Any explanation published or anything?
I can’t see anything mentioned in ZI
Go to your feed on my.zwift.com, look at the Fitness section on the right side. Do you see any new PRs there? They will have a date next to them which will probably help you understand which ride the change happened in. The PRs last for 90 days so an expiring PR will also affect those numbers. It is normal for a zMAP increase to result in a zFTP reduction, that has always been true.
No changes to the calculations have been announced or mentioned by other riders AFAIK. That doesn’t mean it didn’t happen but if it were mentioned in the usual places, I would know.
Did everyone race on Hell of the North again? 
From zwiftpower, it looks like you did ~3.1W/Kg for 5mins in your ladder race on Croissant.
zMAP predicts your 6min ability from your 4-6min data and given the flat profile 4-5mins it shows, I suspect your zMAP will be much nearer that figure than your current zwiftpower 6mins.