There may be 2 things going on.
First, no matter what, a tire is always going to develop at least a bit of irregular wear. It might be a very small amount. It might be huge. It just the nature of the beast.
Why? Because tires are never round and completely uniform. There is always some amount of runout and force variation. In fact, every tire has a variable pitch sequence (tread pattern) - with the exception of some low speed tires such as garden tractor tires.
There is also always some amount of imbalance - even after the tire is balanced.
Put these altogether and these imperfections cause irregular wear - which in turn can cause not only more imbalance, but increased force variation.
Second, because tires grow, they may not grow evenly. They may develop some amount of imbalance and force variation.
Put these 2 together - then add the starting amount of imbalance and force variation - and the result is a tire that requires rebalancing.
But I think most of what happens when a tire develops a vibration late in life is because of irregular wear caused by mis-alignment - and by misalignment, I mean anything less than perfect.
Ya' see, there is no such thing as perfect alignment. Every time you turn a corner, the steering geometry changes and the toe changes a small amount. That's called Akerman (variable spelling). There is one and only one speed for which this akerman is perfect - so, of course, that rarely happens.
Then there is also suspension travel. Suspensions don't move straight up and down. There is either some sideways movement or some angling, or both, taking place.
Did we talk about bushings? They also deflect a bit - even steel ones!
All this turning and suspension travel adds to the irregular wear.