Some people are determined to be low risk by various variables and the TSA computer may granted PreCheck more often to those people than to others. Age, travel patterns, ect ect determine the risk assessment. I've also noticed people/spouses that book tickets together with someone that is PreCheck or a member of a Trusted Traveler Program will have a higher than normal chance of getting PreCheck. They still have to be determined to a low risk traveler but i've noticed this correlation.
Add in wanting to have more people sign up for PreCheck is a good motivator to give these kinds of people a taste and then take it away once or twice.