it means that the account had low 'karma' because of risky factors like 'Nth new account from this IP' and such as this. Then you have some chance to recover it by talking with friends from your contacts or getting messages from someone first. Telegram thinks 'oh, good, ok', your karma gets higher and you can use account normally. But if you join the group as your first action on this 'weak' account instead of doing normal-ish things, Telegram thinks 'SUSPICIOUS', drops your karma, it gets lower than threshold and boom