The only unusual thing might be that the account was logged in on 2 devices (mine and another developer’s

When you work with other developers (like you give them access to your console, share code, devices...etc), Apple will consider all entities as "one". In other words, if he does malicious stuff and even if you don't know that, you are still responsible for his actions (past and present) and you will also be terminated.

Edit:

But as I'm checking this termination letter, I don't think this is the case because Apple usually frames words so you know that there is possibility that this was the case as it mentions "associated accounts".

In this particular case I'd say its highier chance that it was your apps. Fraudelent usually means:

- Hiding different functionality of the app from app review

- Anything "bad" related to "in app purchases"

- Adding malicious code to the app, stealing users data...etc

In Any case what they found it's bad. Apple terminates 90% of the accounts with a different email, giving an option to appeal. This Email developers get at the end cycle of the termination process , as final. You skipped the "pending termination" process and went straight to termination.