Caleb slapped first, and Canelo threw it back. Boxers have quick reaction time and they are able to think fast and react fast - especially these two. Caleb probably didn't want to throw a punch but he got pushed so he probably just wanted to sneak a slap in but wasn't expecting to get countered like that. He got stuck after the counters and it look like it got to him after the altercation. Most boxers throw open hand slaps if they get into it at face-offs so they don't risk injuring their hands before the fight if it's close to the fight date. Mayorga threw slaps and back handed slaps all the time before at press conferences and face-offs. If someone throws a punch then yes they throw punches back, it all depends on what happens at the moment.
This isn't fake between them, their camps has been talking shit to each other back and forth for a very long time now. Caleb has been calling Canelo and his whole team out including his coaches. It's personal between them. This is probably comparable to Jones and DC and thinking their press conference altercation was fake.