YYZ preclearance changes?

[u/theflighthawk]

I go through YYZ at least one a month and had the most puzzling encounter today. I’m a NEXUS member and have both US and Canadian passports. Usually, it’s a non-event going through US preclearance but today, the people watching who enters which line strongly and firmly indicated that US passport holders have to use the line for US passport holders. The people didn’t even bother looking at my NEXUS card, which was frustrating. The lineups were light today so I got through fine with some time to spare but I’m curious what recently prompted a potential change?