Did you check the levels in your tank when you found the fish dead?
If the LR was uncured, it will have a lot of die off, creating an ammonia spike in your tank, which can kill your fish.
Next time you buy LR, ask them if it is cured or not. It should be cured for around 6 weeks or so prior to adding it to an established tank. Uncured rock is great for a new tank that is cycling, but not for an established tank.