This is very rare. I would have to think that something was wrong with the clowns. It's up to the clown if it gets eaten or not. If there is something wrong with the clowns slime coat, it may trigger the anemones nematocysts to fire. If the clown is health, and the tank isn't stressful, you shouldn't lose clowns to host anemones. I would find a new source for your clowns and/or quarantine the fish to insure they are healthy before adding them to the tank. Do you have more aggressive fish that are chasing your new clowns?