You were participating in a mugging pipeline with 2 other people.
Simply put, killem (iirc thats his steam name, I dont have the file readily available rn) was mugging a bunch of people on chars that were brand new, across multiple characters. Then, he would put the spoils into a bank account connected to you and one other person. Then, he would be added to the account on a new character and continue.
This is an obvious case of you participating and benefitting from a guy using throwaway chars that have no value to mug people.
Those accounts will be cleaned out of all custom items that were mugged before you return from your ban.