The biggest reason I've had for tires going out of balance is losing wheel-weights. Sometimes hitting a good bump is all it takes for one of those jokers to fly off and mess your balance up.
Newer vehicles will have factory-balanced and installed tires and balancing is not usually an issue unless there’s been a collision. Older vehicles may develop tire balance issues from excessive miles and wear and lack of maintenance. An indication that your tires are out of balance is vibration. If you feel rough vibration in the steering wheel, the front tires are probably out of balance. If you feel rough vibration in the driver’s seat, the rear tires are probably out of balance.
Tire Balancing is often confused with – but they are two different things. If a tire is out of balance, it can be felt as vibration at highway speeds in the steering wheel (most likely a front tire and wheel), floorboard and/or seat (most likely a rear tire and wheel). Another symptom of a shaky ride is a scalloped or cupped wear pattern on one or more tires.
5. Patchy wear implies the tire is out of balance. Have a mechanic spin and rotate your tires, as this will help equalize the wear. suggests having tires spun and rotated every 5,000 miles, but it’s a good idea to consult your owner’s manual first.