Market conditions, supply & demand!
I usually fly out of YVR to YYZ as I like the 787 and if I am going in J, I rather enjoy it out of YVR instead of the A319/A320 from YEG/YYC.
Fares change on demand
Last summer when the Jays were in the playoff, I went to game 5 and when I was booking the fares kept moving up that day so I finally said enough and purchases two reward tickets and flew into the Island instead of Pearson.