First check that the mat is machine washable and that its backing and wet weight suit the appliance's instructions. A small-looking item can become unusually heavy or awkward when saturated.
Understanding this situation
A single bulky piece may not distribute like an ordinary mixed laundry load. Rubberised, damaged or detachable backing can introduce a separate suitability problem. Adding arbitrary heavy items is not a universal cure, and forcing a spin is particularly unhelpful if the machine is already moving violently or the mat is deteriorating.
A useful next step
Follow the textile and programme limits before loading it. If an ordinary cycle has already stopped with imbalance, use only the manual's normal completion guidance and avoid reaching into a moving drum. For an enquiry, describe the actual mat and backing, not just its apparent size, and say whether normal clothing loads have the same difficulty.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.