The Sky commentators said VAR was going back to look at the approach play before confirming decision so I presume it was for DOGSO as they wouldn't check the APP for SFP. Unless there was a potential penalty or another potential red card beforehand as well of course!
I believe they were checking to see if the attacker was offside, if he was then a different start mode ie indirect free kick to arsenal would have occurred.
Personally i believe the red was issued for dogso but if it was sfp the red card should have stood if even if attacker offside. Pickford/van dijk/ oliver come to mnd.