"Locust" is a reference to the dried seed pods on the branches blowing in the wind - a sound that is similar to the wings of the insect locusts swarming.
Crazy, huh?
From wikipedia: Origin of names[edit]
The common English name "hornbeam" derives from the hardness of the woods (likened to horn) and the Old English "beam", a tree (cognate with German "Baum").