Why does Mullet Fingers put a bag on Roy's head?

Mullet Fingers is a boy of mystery and doesn't want anyone to know anything about him, especially in relation to his protection of the owls' nests. So when Roy comes across a bag of cottonmouth snakes that Mullet Fingers has been keeping, it's just a little too close for Mullet Fingers' liking. He feels like his long-cherished privacy is threatened by someone poking around his things.
In addition, he's waging a one-boy crusade against the building site and so is deeply suspicious of any outsiders. For all he knows, Roy could be one of "them": one of the enemy. So he puts a bag over Roy's head and ties him to a tree. At the same time, Mullet Fingers isn't a bad kid; he has no intention of hurting Roy. He only uses the cottonmouth snakes to scare off the guard dogs. Besides, he soon frees Roy, telling him to count to fifty before turning around. When he's finished counting, Roy turns round, only to see that Mullet Fingers has vanished.