DIAGNOSIS An unbalanced load. The washer's spin cycle is very powerful, and if the weight inside the drum isn't distributed evenly, it will shake violently as it tries to find balance.
FIX 1) Stop the cycle and open the lid (it will pause). 2) Redistribute the clothes manually. Try to mix large items (like towels or jeans) with smaller ones. Don't wash a single heavy item like a blanket by itself. 3) Make sure you're not severely underloading or overloading the machine. A loosely packed, full load balances better than a few heavy, wet items clumped together. 4) Close the lid and press start to let it try to re-balance.
IF THAT DIDN'T WORK If it happens with every load regardless of how you pack it, the suspension rods or springs that hold the inner tub might be worn out or broken. This is a more involved repair, but it will cause constant instability.
HEADS UP Always ensure the washer's leveling feet are firmly on the floor and the machine is perfectly level side-to-side and front-to-back. Use a wrench to adjust the feet, and tighten the lock nuts against the cabinet once it's level. An unlevel machine will walk every time.