The cutting mat is drifting during the cutting operation and is not held in place properly by both of the white pinch rollers.
This concern is caused by one of the of the following:
During the loading process, line up the mat with the small guide mark on the far left side of the machine (rather than loading it justified to the left against the blade adjustment receptacle). The guide mark is highlighted below for emphasis showing where the edge of the mat should be aligned against:
Also check that the right-hand roller is locked into place by having the blue lock lever pointing upward, and that the right-hand pinch roller appears to be fully gripping the edge of the right-hand side of the mat (as shown below):
If using a material size that is smaller on the 12x12 cutting mat, please note that it is still necessary to have the pinch roller in the default position for the 12x12 mat (rather than adjusting the right-hand pinch roller over to the left for a smaller page size).
Please review the following video showing proper loading methods: