The oils in coffee beans are released during the roasting process. Simply put, the longer the beans are roasted, the more oil they release. This oil can be a great thing; for instance, it creates a fantastic crema on espresso.
However, it is a major problem for the burrs in an automatic machine’s grinder. It builds up over time, creating large deposits.
And since the oil is sticky, this creates a snowball effect, causing the deposits to form faster and faster.