My guess is it's the insecticide. I don't know how toxic insecticide spray is to shrimps, but I do know those mosquito mats contain active ingredients highly toxic to fish(allethrin). Shrimps are like underwater cockroaches anyway . That's just my uneducated guess... it just seems to abrupt a die off for anything natural to have happened. Assuming you removed the dead shrimp the days you discovered the dead shrimp... wouldn't die off en masse like that..

CRS too... ouch...