What makes a car tire go out of balance?

A weight falling off?

Unusual wear pattern?

I might have a tire out of balance now, I am going to check it out.

Could be a weight falling off, mud or some foreign object getting stuck inside the rim, or something going wrong with the tire like one of the steel belts inside the tire breaking (that happened to me).

Tire wear (especially if caused by a tire being out of alignment), bad alignment, and other mechanical problems like a loose or worn wheel bearing can also cause symptoms similar to a tire being out of balance.

If it is out of balance you will usually get a severe vibration or shaking at a certain speed.Take it on the freeway or an empty flat road.It will be more pronounced if it is a front wheel.
Did you have a flat and fill it with a can of air? That can throw it out of balance too. Check all your tires carefully to see if you can see if any of those little counter weights have disappeared.
And it could be out of alignment. Check for that very early or late in a empty shopping center. Line up on some straight and flat parking markers, speed up to 20-25 going straight , then let go of the wheel , just release your fingers. If aligned the car should go straight or almost straight for 100-200- feet before it starts to veer . If none of these work, go to a tire dealer, most people do not have equipment to balance or align tires on cars

tires are actually pretty complex items with cords, and belts and stuff inside them.
Tires will go out of balance for a number of reasons including:
[li]Tire wearing in[/li][li]tire weight falling off[/li][li]tire moving on the rim (yeah they can and will move)[/li][li]Unusual wear patterns[/li][li]crap like mud deposited on wheel[/li][li]bent wheel (not really an out of balance condition, but can give the same symptoms)[/li][/ul]
I had a set of Goodyears once that went out of balance like clockwork every 5,000 miles.
One thing I learned from my father is rebalance your new tires after a few thousand miles. Depending on the tire, the difference in ride quality can be surprising.

Back in the 70’s I had a tyre that went out of round. That was fun.

The tire can delaminate interanlly and the steel belts under the tread can shift sideways. Had that happen after 500+ miles of strong crosswinds in Wyoming once.

Years ago, there was a shop here that shaved tires while promising this procedure would double tire life. The theory was that all tires when manufactured were not perfectly round and due to this, the tire would wear unevenly even if balanced. The shaving process would remove 1/64" of an inch or so but the shop explained that the loss due to the shaving would be more than offset by the benefit of a more round tire.

Some people swore by it. I never tried it but it’s an interesting theory.

Yeah…a 40,000 mile tire will become a 80,000 mile tire.

I think we both know why you never went for this one.

Seems to me that a spinning tire in contact with an abrasive surface would act kind of like an inside-out lathe and eventually end up wearing to be perfectly round, even if it didn’t start out that way.

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.