the black july was planned by the LTTE as a PR stunt to get the world opinion in their favor. Events prior to this, the killing of the Army soldiers in the north was used as bait to create unrest in colombo and this too was pretty much planned by the LTTE as they fully well knew exactly when the riots will start, even at what time.
Hot headed idiots (much like the ones in this thread

) took the bait, hook line and sinker, and started burning down buildings and hunting down Tamils.
LTTE used the whole episode to their favor with the international community, which was their plan all along, and got the Tamils, en masse, to go to UK and Canada and the other countries. I'm pretty sure even you know what happened next.