No cause literally, that’s what also bothers me so much and no one ever talks about it
Considering his stubbornness, I think he needs some kind of an excuse to come back. Because I think that he was so flustered for being an Omega then getting pregnant overnight without adjusting first to an omega life. His mind was clouded with fear that he just wants to run away from everything. But even if they didn't accidentally bonded, I'd like to think that they are still going to be together, it would be just a twist.
It bothers me how it wasn’t for Dojun and Byul that he came back. It’s just so that he wouldn’t die from being apart from Dojun. Just wow. I could put up with how Hyesung abandoned Byul because he’s flustered by the unwanted pregnancy, but I can’t put up with this shit.
If they hadn’t accidentally bonded, Hyesung wouldn’t have come back. I think that’s what makes me the most upset. It’s as if Hyesung wouldn’t have return to Dojun’s side if it weren’t for the bond. It’s as if he only did so because the circumstances force him.