could be that female got molted so males are looking for her to get babies. As far as I am experienced, most of my molting issue, they become slow, don't move and then die. (when can't molt)
edit: shrimps are so sensitive to ammonia. so even slightest could kill.
not on topic but it is best for survival and for long run, it is best to wait until Ammonia is 0. I waited 3 months for cycling my ADA