jay's thought is a very good one. crustaceans are definitely vulnerable during and right after shedding, before their new shell has hardened.
another possibility is molting deaths. i've heard of it on a shrimp forum, but am confused about the efficacy of iodine supplements. Some people recommend using liquid iodine supplements to help crustaceans shed (their exoskeleton needs iodine), while others say it is useless. Either way, I use it. You can find it in marine reef (coral) supplements. I just add 1 mL per 30 gal after water changes. I figure it can't hurt :huh: