Any carnivorous creature at that time that lived in the ocean and had a mouth big enough to eat one...they were'nt picky.
The above answer is obviously far too vague and is not quite correct. Instead I offer the following:
Trilobites were early relatives of the arthropods. They had ways of protecting themselves with an exoskeleton and also with spines. Evidently they were not just an available food source for whatever had a large enough mouth and happened to be hungry! Anomalocaris was one of the natural predators of the trilobites and it is also thought to be related to the arthropods. Scientists have found fossilized trilobites that have been attacked and killed by these creatures. The bite marks match perfectly with the mouth of the anomalocaris so it is apparently true.