It is a new rule that they added this year:
http://www.aircanada.com/en/aeroplan...des/share.html
....Additionally, note that this year, an Aeroplan Member, regardless of their Air Canada Top Tier status, can only be identified as an eUpgrade Nominee by one Air Canada Top Tier member.
From a recent thread, some had said they were be able to nominee by more than one Top Tier members. Maybe those people can explain how did they be able to do that.