Incorrect threading, loose upper thread tension, incorrect bobbin case placement and thread tails that are too short are some of the causes for thread bunching up. Also ensure that feed dogs are up and working properly.
Additionally, a mistake such as forgetting to put the pressure foot down can prevent the fabric from feeding through the machine quickly enough, causing thread to bunch up. An unclean machine, blunt needles and poor-quality threads can also contribute to the problem. If everything else has been checked and the issue persists, the sewing machine should be serviced in case of any mechanical problem.