"Skipping the default startup of database" SQL notification for AlwaysOn Cluster
Same kind of message appeared in our environment as well.

What I understand is, as the DB was in AAG, and when the node (secondary AAG group) goes under restart, the DB start-up will be taken care of by AlwaysOn instead of the regular SQL Server 3-phase process!

If the message appears without any secondary node restart, we may have to consider troubleshooting, but in our condition we can ignore that as the message states it is just informational.