for one of two reasons.
Firstly, it may be that the load is not balanced properly or that it is overloaded. Alternatively, there may be a fault preventing the load from balancing. If it happens occasionally there is probably nothing to worry about. A regular occurrence, however, can damage your machine resulting in costly repairs.
If the jumping occurs at random times during a cycle it may well be an intermittent fault. These can be difficult to detect and diagnose and you would be best advised to call an engineer in.
If the fault is happening during the fast spin cycle, it may be that it is caused by the loading. Overloading is an obvious problem but surprisingly under-loading can cause be even worse. Try to balance items when you load. If you put several large towels in with light fabrics then you risk the heavier items bunching on one side of the drum causing an imbalance.
Problems of under-loading can occur when there are not enough items to fill the drum. If there is an empty section it can get out of balance on the spin cycle.
Modern machines do not allow the drum to spin if the load is out of balance. They will automatically try to balance the load but if they are unable to the programme will be ended without a spin or the spin will be aborted and the machine turned off. In either instance the washing will be totally wet through.