D-Barns = Detention Barns.
I can't speak for the all USA tracks, but The Meadowlands has one and stewards can enforce a horse nominated for any races by my understanding for up to 48-72hrs prior to it's respective race, irrespective of the status of the race.
Your horse trains down for that race under detention barn lockdown.
They can do it to any horse, more so, any trainer they have suspicions on as far as I'm aware.