“Donuts are primarily a result of case reforming, specifically involving necking up to a larger bullet diameter”
Wncchestr, When forming I normally neck up first, I have never had a do-nut form while necking up a case, going the other way, a different matter, when I form cases I do not confuse the two, there is necking up and there is necking down. Necking up and or down is not case forming. When I form cases the shoulder is not moved back, the shoulder does not move, it is erased, when I form a case the neck becomes part of the shoulder and the shoulder becomes part of the case body, When forming a case the shoulder that is formed is a new shoulder, the new shoulder is formed from what was the case body.
Back to the questions: Is it flow? or Is it case stretch? or Is it stretch and flow?
Then there is case forming going the other way, as when forming wildcats. An example would be the 30/06 to the 30 Gibbs. The 30/06 shoulder does not move, it becomes part of the case body, the shoulder is formed from the case neck, because the case shortens as much as .035” I prefer to form the 30 Gibbs from 280" Remington cases. 280 Remington cases are .051” longer than 30/06 cases.
Do-nuts, when forming appear at the shoulder neck juncture when forming the shoulder with brass that once was the case body, my favorite forming die and neck reaming die is the 243 Winchester and 308 Winchester. My methods and techniques do not work for others: rational? I have to explain why my favorite forming and reaming dies are my favorite.
The OP ask why do do-nuts appear in his fired cases, I ask what chamber, what rifle.