Some people here prefer to discuss idiot questions like "why the rioters attack US embassy", meanwhile the answer is obvious for every normal, sane and logically-thinking man.
Because the USA headed the aggression against Yugoslavia in 1999, because the US planes bombed Belgrade and all Serbia, because the US tanks came into Kosovo, because the US biggest military base was founded there, because the USA encouraged Albanians during these years and because the USA actuated their so-called "independence".
Maybe next the USA will support Basques in Spain, the Irish in Britain, the muslims and Corsicans in France. And then these people will be wondering - why the rioters attack US embassies in Madrid, London, Paris?.. Oh, it's so strange - we are so kind, we're bringing democracy and freedom, why those strange and silly rioters attack US embassy?..

Absolutely not understandable!
By the way, when are you going to free Basques, Paris muslims and Corsicans?