What did happen to make Lauren move away from Sean?

They broke up for a number of reasons: Sean's job was taking a toll on their relationship, thus creating distance between them, then she told him she had an affair and fell pregnant, resulting in Sean not taking the news so well, calling her names and almost hitting her. Here are a few quotes from the book:

They started drifting apart due to Sean's job.

She told him he was becoming hard, intractable, reductive in his thinking. And he didn’t respond because there was nothing to argue. The question wasn’t whether he’d become those things, but whether the becoming was a positive or a negative.

Lauren had an affair, and fell pregnant, but it's not clear from whom. And that's what makes it even harder for Sean to accept.

Still, he probably should have seen the affair coming. Maybe he did. And maybe it wasn’t the affair that truly bothered him, but the pregnancy that followed.

Sean discovering the affair she’d had with an actor, of all things, starting to ask her, “Whose kid is it, Lauren?” and Lauren coming back with, “Take a paternity test, you’re so worried.”

Sean didn't take the news too well, and I assume they broke up at this point.

And had Sean hurt her? Well, yeah, of course. When he first found out about the affair, he’d come damn close to hitting her, pulling back his fist at the last moment and putting it in his pocket, Lauren seeing the urge in his face, though. And all those things he’d called her. Jesus.