I've connected through Toronto Pearson International Airport to the US twice this year, and both times I was directed to the Nexus lines, even though I'm not a card holder.
I scanned my passport as usual, showed the receipt to the agent before customs, and they pointed myself and a handful of other travelers to the Nexus line. After we went through customs, the agent before security somehow knew to send us to the Nexus security line, as well. I'm guessing it was because the customs officer stamped our boarding passes.
Does anyone know why the agents do this, or what the criteria for it is?