Reason behind COD:M account banning may varies to the Call of Duty: Mobile Security & Enforcement Policy. Please check out this article https://activision.helpshift.com/hc/en/3-cod-mobile/faq/68-call-of-duty-mobile-security-enforcement-policy/.
And if you need more details related to your account banning, please submit a request to Activision Support team from this page https://support.activision.com/options