When you see leaves in your screech-owl box opening, it’s nature’s way of telling you a squirrel (and possibly a squirrel family) is living there.
If it’s still early in nesting season, best to bang in the box with a broomstick to evict any squirrels, then clean out the box (wearing protective goggles/glasses to keep the dust and gunk out of your eyes). Repeat as needed.