Sounded like somepony's playing pranks in the Watchtower to Lily. Maybe one of the all-unicorn 'corn-field' battalions stationed in the Watchtower decided to have some fun messing around with the First Air Division's pegasi. There were an awful lot of illusionists in Canterlot's anti-air gunnery brigades, and they were all mischief-making rascals, every last one of them.