It wouldn't be for dating/marrying an 'unbeliever' - that would be a 'marking' matter, NOT a JC, providing the courtship was a 'clean' one.
Therefore the most likely reason would be for entering an 'adulterous marriage'.
Shepherd the Flock of God, page 130
Adulterous Marriage
If a divorced person remarries and he was not Scripturally free to do so - in other words, if adultery and rejection by the innocent mate had not occurred - he has entered into an adulterous marriage. In Jehovah’s eyes, he has married someone while still bound to another. Entering into such a marriage would call for judicial action.
If the disfellowshipped one was eventually reinstated, the elders would be very cautious in extending any special privileges. He could share in the cleaning and repair of the local Kingdom Hall. He may eventually give student talks in the Theocratic Ministry School if his doing so would not disturb others. However, he would not be assigned to help with literature, accounts, magazines, attendants, or similar privileges in the congregation as long as the innocent former mate is alive, unmarried, and has not been guilty of porneia. - w83 3/15 p.29.