If you're going to use pine try to make sure it's a very low dust pine.

These rats were born at your home but were the parent stock from pet stores? Pet store rats tend to be very inbred or indiscriminately bred so you'll get more sudden deaths until you widen up that genetic pool a bit. I used to have rats drop for no reason quite a bit when we first started out but it's a rare occurrence now. Still happens here or there but not like it used to back when we were working with a lot of pet store purchased rats and their offspring.