By adding live rock to the tank you started a cycle all over again. What this means is that the die off from the shipping prcocess has started an ammonia cycle, just like when you first started the tank. The fish most likely died from ammonia poisioning. When you want to add any new live rock you have to cure it before you put it in an already exsisting tank. What this means is to let it sit in a container of some sort with a heater and a PWH and let the cycle tank place in the container. Monitor the parameters wait for curing to be done then add to the tank with minimal damage. This is why you add all the liverock you want before you put in the livestock. Hope that helps.