My understanding is that the upper fuselage is considered a critical surface on all aircraft that have a "T" tail type design. From obeservation I've noticed other aircraft such as the ERJ 145 with a similar procedure. I believe the air passing around the fuselage will pass back and over the horizontal stabilizer of a T-tail aircraft at high angles of attack attitude. Any frozen precipitation might disturb the airflow and have an affect on the airplanes pitch an stall characteristics. I suppose conventional tail airplanes like most Boeings or Airbuses don't have this issue.
I think the packs being off is just to stop sending glycol vapors into the cabin as the fluid is blown back into the packs. I wouldn't think that would make a big difference as it's going into those intakes anyway and once you turn the packs on its going to do it anyway, perhaps it's not as bad?