Does your stand mixer have a soft start feature?
Some mixers have a "soft start" feature. The mixer may hesitate momentarily when turned on. The speeds will then increase gradually to the selected speed. This is to prevent flour and other ingredients from flying out of the bowl.
Did something fall into the bowl while mixing?
This can happen if something hard fell into the bowl while mixing (e.g., a measuring spoon, the attachment hub cover or hard ingredients) Most likely a gear in the mixer motor has become stripped or damaged, which would require repair or replacement.