Database 'template1' vacuum

Hi!

I've encountered something I've never seen before, and I am hoping someone can help me understand what's going on behind the scenes with this vacuum/xid behavior.

ERROR: database is not accepting commands to avoid wraparound data loss in database "template1"

This error popped up right after I restarted a database that had just finished running a vacuum freeze in single user mode for the postgres database (the only database we use).
(That was in response to: ERROR: database is not accepting commands to avoid wraparound data loss in database "postgres")

I'm confused about the need to vacuum template1. All of our activity is in the database named postgres. Do all of the databases share the same set of xids? If they do, why would the single user backend vacuum not handle all of the databases at once? If all databases don't share the same set of xids, then what could be going on internally to cause template1 to run through so many xids?

(This is running on 9.5.3, if that makes a difference.)

Thanks!

Natalie