The cause of a tire going bald in the middle of the tread is over inflation of the tire.
Too much air in a tire can cause the tire to wear out faster in the middle and cause the tire to become bald in the middle.
Over inflating tires can also lead to tire blowouts as well.
When putting air in tires you want to put only the maximum amount of air in your tires that is recommended by the tire.
You can look on the sidewall of the tire and see what the maximum tire pressure should be and it's usually between 40 PSI to 55 PSI.
If you put too much air in your tires you should release some of that air.
Having too my air pressure in your tires cause the contact patch of the tires to shrink and the center of tire to take most of the load.
That then leads to bald tires in the middle.
If the tire pressure is too low the tires tread wear on the edges of the tire will typically indicate inflation pressures are lower than specified.