Drivers may think they need to honk to let you know they are behind you. (wrong). Or it could have been the truck letting the SUV know it was not safe to pass.

If they are passing over the center line they should have their turn signals on. Bet he didn't.

Could be that the truck slowed down because he didn't have enough visibility of the road ahead to pass giving sufficient clearance (6' for a commercial vehicle).

I wish bike riders and vehicle drivers were required to pass a written test including bike safety rules when renewing their license (and that bikes operating on streets required an operator's license). It could easily be done online.