Luck & strength. Rotator cuff injuries become more common as we age for several reasons. The rotator cuff tendon thins as we age and the bony architecture becomes more narrowed. This can lead to impingement and tearing of the rotator cuff tendon. In order to avoid injury, it is important to maintain strenght and flexibility of the shoulder joint. Try to avoid things that aggravate your shoulder.
Be smart & lucky. While there are some variables that you have the ability to control (such as how you lift, what activities you participate in, etc), there are some variables that you cannot easily control. This includes normal aging of the tendon and genetics.