Asalam o Alaykum
ahnaf- com
you can find the reason of shia sunni fights here
there is allot of shia videos where they are cursing companions of prophet and umhat ul momineen this is the basic reason of fight, when they curse them they face sunni protest.
i can see mr aqeel and some other shia's are trying to hide the facts but you can see videos on this website and i hope there will be no more questions in your mind after watching that videos.