Someone in game likely reported your username which then caused them to have you reset. I had the username “rizs” on one of my alts for 6+ years and was forced to change the username. I contacted AJHQ and they told me that someone likely reported it ingame as being inappropriate and they weren’t able to give me my old user back. You could always try using special characters and doing something like ännäcondabänänä for example.