What made the cart go loose?

From the text:

Inside the base of Freddy's Free Fall, hidden from view, the cable that lifted Cart No. 2 had, for the last few months,
been scraping across a locked pulley. Because it was locked, the pulley had gradually ripped the cable's steel wires—as if husking an ear of corn—until they were nearly severed. No one noticed. How could they notice? Only someone who had crawled inside the mechanism would have seen the unlikely cause of the problem. The pulley was wedged by a small object that must have fallen through the opening at a most precise moment. A car key.