It's possible that different species of Cleaner Shrimp will snip Antenna as they fight for territory. Fire Shrimp are one species of Cleaner Shrimp and I'm just guessing that you have another species in the tank as well. They will get along fine one they claim their territory.
Don't worry, the antenna will grow back after 1 or 2 molts as long as they are well fed and have good water.