Not OP, but my understanding reading this.

OP had DL status. Someone went to all of UA, Alaska and AA applying for status matches and presented OP's DL status as their credential to participate in a status match with these other airlines. Somehow DL was alerted to this and therefore suspended OP's account while they investigate.

How DL became aware of this is unknown, some possibilities that come to mind:
1) Airlines have some type of system to verify the status level of match applicants with other non-alliance airlines,
2) The name of the person applying for the status match didn't match the OP's, this would raise a red flag. One (or multiple) airlines could have notified DL this was occurring given the airlines, as a whole, have an incentive, as a whole, to detect and prevent fraud. IMO this is the most likely scenario, as I suspect corporate security at each airline knows corporate security at the other airlines and would maintain some level of collegiality even though the airlines are competitors (i.e. if I work at UA but suspect someone is trying to present me fraudulent DL credentials, I would FYI DL, because I would want them to do the same for me if the roles were reversed).
3) Someone came into a copy of OP's credentials showing DL whether that be via acquiring access to their account, or some other unknown method, and attempted to sell that credentials online as proof to use towards a status match with another airline. All airlines aggressively monitor any attempts to sell/barter with their products.