It is likely you ran something in the background of the app or where part of a group who abused a bug for example in leviathan jumping through the small bugged wall opening to cheat across the map or you did something else.
Hard to say but bungie has a no discussion policy and its done by people not a system so there must be a reason.
If it was without reason i hope you get access again but i am assuming and i am just saying i assume here ok... i assume that you did something wrong and you cannot exactly hide it and ask for community sympathy to build as they had a reason most likely. Kinda like how other game companies do it but here they are a bit stricter.
Sorry for your loss tho, who knows maybe its a temp ban ?
note: personally i question random groups when i join them to make sure they arent planning to use any exploits or bugs just in case they could get me into trouble so i walk a clear path. In a way their no-discussion of ban policy makes following rules simpler and less vague only time i use them are when they say its ok as all do it etc. ^^