What happened in Vegas?
Before the show
During the meet and greet Zayn and Liam were completely fine with each other and in the pics they were together and smiling.
After that Zayn was seen outside talking with Paul and another man. Due to their expressions it's appropriate to say that they were have a not nice conversation. See that Zayn's hand is perfectly fine.
Gordon Ramsay posted this pic from his dinner with One Direction at his steakhouse in Las Vegas before the show. Zayn's not in the pic, so he wasn't there.
During the show
After the opening act the fans saw Danielle and her sister coming from the backstage and as if it wasn't enough it was her birthday eve. X
As soon as the boys were on the stage, everyone saw Zayn's hand was hurted, the media said he got hurted while working out.
Zayn ignored Liam all the time, but with the other boys he had no problem to interact.
At the end of I Want the boys used to hug and bow, Zayn was always next to Liam but that night he walked away from him when Liam tried to hug him.
The most iconic ziam moment of the night was at the end of More Than This, when they had to sit on the couch and Liam sang his part directly to Zayn while Zayn ignored him. X
The theory here is that Zayn didn't know Danielle was going to be at the show, so when he found out he had an argument with Paul and the other man and then he hit a wall out of anger.
Liam didn't tell Zayn that Danielle would be at the show so he was upset and that's the reason why he was ignoring him all the time and Liam seemed to be very repentant.
The next day in Phoenix, during Liam's solo in I Want, Zayn was fondling Liam's face, so they were back to normal.
