This article misses a very big reason. It's a scheduling type problem. They rarely have good systems in place to deal with taking bad trollies out of service, so they stay in service and sit at the front of the stack, because some people notice and reject the trolley but they generally find their way back onto the stack. If you always take the first trolley of the stack and regularly find it is busted it is probably because you are often taking other people's rejected trollies