The following page of the Wasteland 3 guide provides information on companions' deaths. Read it to learn how to avoid it.
Each character has its own condition bar (CON) which defines its health. When it falls to zero, they will lose consciousness. If this occurs during a fight, a turn counter will appear over the character. It determines how much time you have left to revive that companion. When it reaches zero, the character dies.
After each resurrection, the character receives various injuries that affect the statistics negatively (you can check them by entering the inspection tab). These injuries can be healed by using an injury kit or by visiting a doctor.
Injuries are significant in terms of the chance of death. A character with injuries doesn't renew its resurrection time limit. This means that if you have resurrected a character after two turns, the timer won't restart when that person is knocked down again. You will have to react faster. If you don't treat injuries, you can get to the point when that character will die immediately after losing the entire condition bar.