DB STARTUP command active for an online database
As confirmed by you the SQL server is of enterprise version and database was not starting after restore so the recovery process can utilize fast recovery.  In enterprise edition the database would come online after second phase (this is called Fast recovery which was introduced from SS 2005 onwards) and you would see database as online but still internally third phase, the UNDO, phase is going on and would be accessible completely only after third phase completes. What I mean by completely is For fast recovery, a query may bump into one of the locks being held to allow fast recovery – in which case it will have to wait for that lock to be dropped as UNDO progresses

I suggest you also read 


When is fast recovery used
3 phases of recovery