Your hives may be hot because they are over crowded.
30-40 yards away is pretty aggressive. It may be more than over crowding.
My hives are over crowded now, mainly because i can't get into them with good weather the same day I'm off work. Some are mean with lots of guard bees. Some not so mean.
I'm getting hit 5-10 yards out, first sting yesterday right before a big storm.
I've had problems with robbing as well.
When I smoke they calm down, just smoke in the yard helps.