Some people just dont like being overtaken. In a jam some people pull over to let you through as they realise you wont actually hold them up, some try to block your path because they think you should queue as well.
Its not worth argueing with another driver in situations like this.
As an aside, I also drive a van, rear windows obscured, and its amazing how many motorcyclists drive right behind out of view of the mirrors. If they check their mirrors before overtaking just as you decide to overtake it could end in tears. If you cant see the driver, they cant see you. Same for the passenger side if there is no window in the sliding door, the mirror provides a view to the rear. I've got one of those wide angle mirrors stuck on, but the visability isnt great. Even if the van has got a window in the sliding door, there might be stuff in the van obscuring the view.
Its not worth argueing with another driver in situations like this.
As an aside, I also drive a van, rear windows obscured, and its amazing how many motorcyclists drive right behind out of view of the mirrors. If they check their mirrors before overtaking just as you decide to overtake it could end in tears. If you cant see the driver, they cant see you. Same for the passenger side if there is no window in the sliding door, the mirror provides a view to the rear. I've got one of those wide angle mirrors stuck on, but the visability isnt great. Even if the van has got a window in the sliding door, there might be stuff in the van obscuring the view.