"I agree with tribal but wonder why in the UK, the aggressive behaviour seems to be more common amongst football than say rugby supporters. "

Often the fanbases have been different. For instance in Scotland football was the urban game where working class lads would go off to blow off steam. Rugby in the cities was played and watched by the more well off - or at least those from less disadvantaged areas. The only Scottish exception being my area in the Borders where rugby is the main working man's game. There are other big exceptions elsewhere too - for instance industrial south Wales is rugby mad and rugby league is very popular in the northern English cities. So like all stereotypes it doesn't fit the bill but I think it is safe enough to say that a young lad looking for a rumble in a big urban area is more likely to find it at football as he is more likely to go to football - and likewise anyone else likely to be drawn to that would know where to go. It is cultural too. For a while in the 80s a minority of England fans seemed to plan trouble. It was deliberate. On the other hands Scotland fans had been in trouble more for general drunkeness and over exubirant celebrations. The Scotland fans never developed the lets go and deliberately cause a riot habit of the 80s. Probably perversely because they saw England fans doing it and didn't want to be tarred with that brush. In fact they then seemed to make a real effort not to cause trouble. Never stopped them at club level of course:-)