ambush - To suddenly attack someone after hiding and waiting for them:
The brigade walked quietly to the hidden spot on the hill, patiently until the enemy walked in the valley below, so that they could ambush the unsuspecting enemy, who were taken by surprise.
arbitrary - Based on random choice or personal whim, rather than any reason or system
She felt that when the judges chose her competitor as winner of the debate, that the decision was totally arbitrary, and didn't recognize how well she had really done.