Imagine you had a fenced off field in which you kept a whole bunch of cute fluffy bunnies. And a jaguar. One day you add a cow, and the next day the bunnies are all gone. It's possible, in the strictest sense of the word possible, that the cow suddenly changed its nature entirely and ate all the rabbits, in spite of a history and feeding style that is completely contrary to that behavior. It's also possible that the jaguar got sick of seeing all the fluffy tails bouncing around and decided to do something about it. I know where I'd put my money, even if the jaguar didn't normally eat bunnies.
Just to be sure, your cow is a cow, right? Er, bamboo shrimp, I mean. Largish shrimp, filter fans for claws, mostly just sits there with its filters extended?