Always On -> Manual Failover always results in the former primary "rebuilding"

When i do a failover to swap the primary / secondary nodes, the secondary successfully takes on the primary role but the primary (now secondary) is "critical". It appears to be resynchronizing the entire DB (it takes like an hour to become healthy)

Is this normal? When I failover again to revert the roles to their original nodes, it works without issue, it doesn't resync the entire DB again. Both nodes report healthy.

Whats going on with the primary node thats causing it to resync after a failover?