Why did he act like this?

Maybe he's afraid or something like this.'Give him a chance if you enjoyed his company.

You gave an asshole exactly what he was online looking for. Now he is moving on. That is all.

It is also easier to hide the fact that you are involved with someone. It is simple to call from another location (Hey, I am going to the store, want anything?) And you can text from the same room, even the same BED, as your
SO, to lay the foundation for a quick "piece." This is is precisely what my last "mistake" did.