I can confirm that some kind of forced upgrade is going on.
I got the same upgrade dialog as @Shamino, and dismissed it in the same manner. LittleSnitch showed that something was downloading at full bandwidtch. I checked their Network Monitor and found that nsurlsessiond was downloading from updates.cdn-apple.com. I killed (-9) nsurlsessiond, and it was immediately restarted.
After a while I checked Activity Monitor, and a process named com.apple.MobileSoftwareUpdate.UpdateBrainService was using multiple cores and a lot of virtual memory (I think I saw 390G, but that doesn’t make sense).
I decided I’d had enough, and I didn’t want to be forcibly upgraded, so I shutdown and restarted the system. I haven’t seen nsurlsessiond or UpdateBrainService yet.
Not sure what is going on.