I am so sorry that poor Hamlet has passed to the bridge
Sometimes animals are born with some little hidden defect inside that is invisible to the naked eye and this may suddenly mean they die. It is like that in humans too. Sadly some are taken sooner than others. There is no way of knowing but at least he seems to have gone quickly and without pain which is merciful. I know you want to know what happened but sometimes there is nothing anyone could have done and it is knowones fault and you just need to accept that then you can let his memories make you smile again. RIP Hamlet