It is theorized that cats tend to hide when they are sick in an attempt to reduce stimulus. You may experience a similar condition when you are sick, where you do not want loud sounds or bright lights around. Running away may seem like the only way to avoid ordinary stimulus for a cat.
When a house cat is very ill it will often leave or hide in order to seek solitude and eventually death. Many of these grave illnesses can be treated if the cat is taken to a vet before it is seriously ill.
This hasn't been my experience over 40 years of having cat companions, but like humans, sometimes when their time comes they seek peace. I have found sick cats seek companionship.
