Well, Vulpera are part of a Horde only faction and the Alliance has directly attacked the Vulpera so... it's not that big of an earthshaker that they joined the Horde, in this case.
While the reasoning why Nightborne join the Horde is a bad joke, they do have a lot more in common with blood elves that night elves. There's a reason they didn't join them originally (i.e., when the Legion forced them to bubble up). As for Mag'har, welp... it was an asspull. It's okay, though, but it's not shocking it came out of nowhere. Only other alternative was to forget Draenor existed and recruit outlands mag'har, which would have been okay, too.