It sounds to me like the truck driver did the..."oh my god there's a cyclist so I better slow down real sloooww and pass him on the left (ridiculously wayyy to the left)" routine while not paying attention to the maniac SUV driver (soccer mom on her cel phone) coming up from behind and now for some reason is going to try to pass the truck that is now weaving into the other lane....thus causing the SUV driver (soccer mom) to lean on the horn as if to say "WTF"!!!!

I don't know what I would've done in that situation but if I had to ride on a highway I would have mirrors like the other folks have suggested.