What makes timing belt loose

What is your question?

My car was shaking a lot when you got over 60 but only when giving it gas. Other than that it ran fine. One morning it wouldn't start too timing cover off and I have a Hyundai Santa Fe 3.5. anyway, timing belt is not broke but where it dips down between right and left cams and goes around a wheel or pulley be it's really loose. What caused that