Possible reason for why Dale picks on Bill
I would imagine that when they were in high school and Bill was a football star that Bill was getting the star treatment from everyone. No doubt it went to his head. Meanwhile, Dale was the lowly towel boy and rather skinny. I'll bet Bill probably picked on Dale a bit. As they got older, things began to go downwards for Bill, Dale remembers the bullying and has been getting his revenge ever since.
Annoying the world since 1960!