This phrase was used in the Vietnam war. The Vietnamese would camouflage themselves and hide in the trees and attack the
Americans (which we 100% deserved.) the
Americans were not used to this type of guerilla war fare and were very alarmed. They would yell “they’re in the trees” to warn the rest of their squad about the attack from above. Now this is a phrase that angsty teens say to express a sudden attack, or they just yell it out the
window at unsuspecting pedestrians.