Now, i will admit...i am a flight attendant for a DIFFERENT airline....so i am not sure of Delta's policies...but i will tell You what OUR policy is and it may be very similar and explain some things...

at my airline...commuting flight attendants (flight attendants who choose to live in one city and work out of another) are NOT permitted to board ahead of other passengers.

HOWEVER....

DEADHEADING crew members ARE specifically told to pre-board or board as soon as they get to the gate...and here is why...

a DEADHEADING crew member is traveling in a regular passenger seat, being paid, and under intructions from the airline to be on that flight so they can get to the next city and work another flight.

The Deadheading crew member may be working a departing flight shortly after the arrival of their arriving flight....

If the deadheading crew member is boarding a full flight and their bags have to be checked, that could possibly delay the crewmember from getting to the next gate to work their assigned flight.....ultimately delaying the departure of everyone onboard and disrupting the on-time flow of the aircraft they are working for the rest of day.

Even more disrupting is that the deadheading crew member may have to check their bags and they get lost...that bag contains their uniform (yes, they can wear the current dirty one) but also their flight attendant manual and flashlight which, per the FAA, they are PROHIBITED from flying without....so now you have a deadheading flight attendant in position to work a flight but they do not have the required equipment to work the flight and NOW the flight they are working is going nowhere.

If it was a DEADHEADING crew member you saw board ahead of other passengers and take a passenger seat, they were most likely boarding (possibly per company requirements like at my airline) to guarantee their bags DID get carry-on space onboard the aircraft.

In fact, at my airline, if we have to board the aircraft late and our bags have to be checked, we are authorized to go down to the ramp ourselves to claim our bags. That is how important it is for deadheading crew bags to be claimed ASAP.

Again, if this was a commuter....going to work on their own free will on a non-revenue, space available pass....this is possibly questionable. BUT....if this was a deadheading flight attendant traveling on scheduling requirements...this could explain why they boarded early.