Why destroy Laketown's bridge?

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Originally Posted by alatar

..and it had the wit to attack Frodo Ringbearer first, knowing that it too could be a Ringlord if it claimed the Ring..

Maybe it was just a mistake from the beginning? Through the dark and murky waters the Washer might have mistaken Frodo for a mop with that wooly hair and all. He was merely trying to clean up his pond.