The majority of belts for washing machines and tumble dryers break for the same cause: because they are placed under an undue stress or friction.
You will see that your belt has been submitted to undue friction because the edges of the break are burnt. The points are slightly curved inwards and the slits have been frayed.
Undue friction occurs when the motor turns but the drum remains still. When this happens the non-movement of the drum prevents the belt from moving and the pulley of the motor burns the belt to the point of breaking it. Why does the drum get stuck? Blocking the drum may be caused by the machine breaking down (because of wear, or the bearing breaking, the axis wearing out ...). However, there are two principal and frequent reasons for the drum blocking: overloading or because a piece of a garment pokes through the internal part of the door. The first of these reasons, overloading, occurs when we wash too many garments which absorb a lot of water (for example towels). The high weight of the load causes the pulley to slip on the stationary belt and ends up burning it. The second reason, the sudden braking of the drum, normally happens when a stiff garment, normally a shoe, trainer or slipper, gets into the glass part of the door creating a wedge effect. This wedge suddenly jerks on the drum while the pulley continues exerting force, turning and inevitably the belt breaks.
The solution in both cases is easy: make sure you do not introduce too many heavy garments in the drum (towels, quilts, etc.) and avoid washing sports shoes or similar garments. If you follow these two tips it is probable that the belt of your washing machine or tumble dryer will last a long time, as it is a piece that suffers little wear and tear from normal use.