The only time Bungie will change your name is if it violates the code of conduct. In this case, your name will be changed to 'Guardian####'. If your name was changed to anything else, and you did not do it, then someone has access to your account.
You should secure your account(s) ASAP. There is no refund for a used name change.
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The name change is NOT used. I signed into bungie.net to see if I can check my triumphs and when I logged into the game it gave me a message saying that the ##### changed and then I noticed my whole name was changed. Something about CrossPlay and it being linked to my Epic, but I disabled Epic and yet it remains.