I honestly don't know. A friend suggested that it may be due to the C40 recall, which I don't think is the reason, as there were still some C40s out and none have been taken out of service as far as I know of.
The last time an artic ran on the Bx32 besides today was exactly 2 weeks ago, and before that, I believe it was 3 years ago, in 2012. Yeah, artics on the Bx32 are really uncommon. The one 2 weeks ago ran right behind another bus, and both of them were pretty empty... Today, there were a bit more people, but the bus was still fairly empty. The artics don't really need to run on the Bx32 from what I've seen, but it sure as hell is interesting to see them.