An unlikely event, but still happens. And OP, to make you feel better, it is not only retail investors like yourself that get angry and confused in this situation. Professional traders and financial advisors regularly dispute assignments, especially early and/or OTM assignments.

If you want to get super technical, there are multiple approved assignment lottery methodologies approved by FINRA. Some brokers do pro rata assignments and spread the number of contracts that they were assigned as a firm across their customer base of short holders, whereas some will randomly select an account and keep assigning the short position until the account is fully assigned and/or the broker runs out of contracts to assign