Jimin and Jungkook were childhood friends who became a couple pretty soon as was obvious with how much they loved each other. Jimin had gotten a job opportunity in Seoul and Jungkook was equally happy about it till his friends convinced him to mate with the omega before Jimin left, claiming that other alphas in the city would try to woo an unmated omega, that an unmated omega was not safe in big cities.
Jimin and Jungkook hadn't had sex till that point. And they had decided to wait till they are both established in their careers before being bonded for life, especially since they were both going to separated for a while, and it would physically hurt them to be away from their mated partner. Jungkook tried to push away all the negative thoughts away from his mind but his friends constantly talked about it and having been drugged by them, he did the worst thing he could do to his omega, without even knowing it. [the drug was known to make an alpha get his rut, lose all sense of rationality and have amnesia of the events the next day]
The alpha forced himself on his omega and forcefully mated with him. Jimin knew this Jungkook wasn't in the right mind so he never told Jungkook. Anyways he was leaving that day, so he left heartbroken and Jungkook had no recollection of what he had done.
Jungkook couldn't join Jimin in Seoul because of his exams in the next few months and Jimin didn't tell him about what happened because he didn't want Jungkook to feel guilty and spoil his future. He decided to tell Jungkook about the bonding after the alpha's exams but unfortunately Jin beat him to it.