Why was the balkan powder keg such a danger to set off the alliances?

Because of the tensions in the Balkans and the consent fighting from both sides. The spark that ignited the powder-keg was the assassination in Sarajevo of the Archduke Ferdinand and his wife Sophia by Gavrilo Princip. He was a member of Narodna Odbrana, a Serbian nationalist organization. 

Look it up somewhere else don’t be lazy kid