The same problem occured to me yesterday. In one moment I was working towards a triumph, in the other moment my screen turned grey and I was banned for no obvious reason. I MIGHT have an idea what could have caused this, but I'm not sure, because there was no further information except for that which you find on bungie's 'ban' page. There were several issues beforehand with both of my connection to the servers as well as graphic performance, so I ran both a test and a scan with Norton Security while Destiny2 was running. Maybe the game misinterpreted the scans as a hack or such? I will definitrly take care that in future no scan will affect game sensitive data (although it does not seem likely to get my account back. Lessons learned, in that case.) I'm quite in shock that there seems to be no possibility to reason for those who might be falsely accused of breaking the rules