I did a little research about why bulls buck and found that this is something that is not not readily discussed among the bull riding community. Its never shown on televison. The only information on the web is curious people asking question and the bull riding community saying nothing bad is going on. I did receive one reply from the bull riding community. Here it is verbatim

"there is NOTHING even touching the bulls testicals, you can't make a bull do anything, the flanks are nothing but a peice of soft cotton rope tied litghting around the bulls flanks, it's nothing but a very slight irratance, the bulls try to kick it off, like a rubber band on a dogs leg, some bulls buck, some don't, you can't make a bull do anything he doesn't want to do, your talking about 1800lbs+ of pure muscle, it's simple, if someting was tied around your nuts would you jump up and down or curl up in a little ball and cry, simple answer. bulls buck when they feel good and only because they want to. hope this answered your qeustion."