I suspect people cheat on their spouses for many different reasons. Maybe he didn't expect to have feelings for Erin, or maybe he was just a player. It could also be that Erin was in an emotional phase of her life, and she was swept off her feet by him. They both seem to love horses, so IMO, maybe that's where they first bonded at. I suspect that Erin was much different than his wife- maybe softer spoken, different conversation, interesting, new, full of life, etc....and she paid attention to the "cowboy" that was good with horses and was friendly with all the ranch hands and kids. Erin probab ly didn't call him things like "stupid" and he may have actually felt like a man with her. IDK, this is just my opinion.
I know most people think that maybe Lee killed her because she was pregnant and he was scared of his wife finding out. Maybe it was the other way around. Maybe Erin went out there that day to END it with Lee. I know she told her friend she was going to celebrate and maybe she was. Maybe she made her decision about her life. She COULD have told Lee "you can still see the baby if it's yours, but I am going to stay with Jon and work it out" and Lee lost it.