Answer:
I imagine that the Zebra mussels are not predators of the Unionod Mussel, so it must have something to do with a struggle for resources. If the population of Zebra Mussels increase, the Zebra mussels will use up and essentially hoard resources like food and the Unionod mussels will no longer have access to what they need to survive. I hope that this helps a little bit!
