Seems otters, like most mammals, turn into major psychopaths when anything gets too close to their new offspring. I recall a similar incident from what I think was the Snoqualmie River in recent years, about the same time of year, where a mother otter attacked someone who was just playing in the river nearby.
Like bear cubs, baby otters may be cute, but if you see one, you had better start looking for your escape route, and fast, because momma ain't far away, she's on a hair trigger, and she's watching....