I had literally just gone through that same point just before this happened!
I have a feeling that the truck was making a left turn from Spadina onto Lake Shore Eastbound. The light has been changed so that the streetcar now has priority and goes through BEFORE cars can make a left. The left turn cycle comes at the end just before the light goes red. Many cars begin to make a left turn, assuming the left turn light will go on, when that is no longer the case, and hasn't been for months now.
BUTTTT.... This morning, I waited 3 light cycles, and the left turn signal failed to turn on at all. It caused a massive backup along Spadina past Fort York Boulevard/Bremner. I think there may be a defect with the signal, and wouldn't be surprised if cars were just running the red because of that issue. I nearly did it this morning too because the left turn signal simply wasnt working.