The Bologna rivalry isn't so much based on sporting incidents, but the Bologna supporters are twinned with some other of our rivals. The Bologna rivalry is the one that seems the most chaotic and irrational, but it's definitely there. I attended Juve - Bologna during the Serie B-season, and there was a mass fight outside the stadium. The following season a Juve fan was put in a coma by 3 Bologna ultras (the Juve fan was a father walking hand in hand with his son, doing nothing to provoke the Bologna supporters):
http://www.gazzetta.it/Calcio/Primo_Piano/2008/10/30/tifoso.shtml
If you want more details about the Bologna rivalry, I can talk to dat friend of mine. I've never understood it either, to be honest, I just know it's there.