Sean Connery once defended it being ok to slap women with an open hand. At the time, I didn't necessarily disagree with him. If I came home to find my wife getting railed by 3 dudes, she'd almost certainly get slapped. I wouldn't be proud of it, but she'd certainly be deserving of it.
Bill Burr also has a stand up piece about the reasons for hitting a woman. He clearly states that you never do it, but that to say there is never a reason is misguided at best. There most certainly are reasons... You just don't do it.
All that being said. The wife in the video was annoying, and refused to leave him alone even after asked. I don't think it deserves a slap, but I certainly wouldn't crucify him for it, if was just a slap.
But she's pregnant. Now the annoying and whining on her part starts to make sense. Her mood is swinging all over the place. She's starting to show, and certainly feels self conscious about it. It doesn't completely justify her actions, but at least now I can understand where they're coming from.
His reaction was to slap her. A pregnant woman. In front of the kids.
Slapping her may have been excusable, but it was in front of the kids. That makes it inexcusable. Slapping a pregnant woman is going way too far.
Debating the how's, and why's is fine if you're trying to search for excuses, or somehow justify his actions. Many people aren't looking for that, and therefore don't bother.