I would not worry about the rooster, sometimes there is genetic defect, that is not severe when little, but the body cannot keep up with as the bird grows. I WOULD be concerned if you lost several more birds or a lot of birds began to become sick or less active.
Chickens do frequently die. It is unrealistic to expect them all to live. I have had deaths with in a few days of hatching, then right around 8-10 weeks, and then between 2-2.5 years. Not huge numbers of deaths, just occasionally, but over the years, enough to notice the pattern.
Single deaths, just fine one day, and dead the next are not real uncommon. Granted I have always had hatchery birds, or barnyard mixes, and they are not bred for longevity.
When you need to be concerned, is if the flock is sick, or several die. I will admit, that one spell I lost one bird a month for three months running, I was getting worried, but never lost another one.
Sorry for your loss, but if you keep chickens, keep a flock, knowing that birds will be added and removed from the flock.
Mrs K