Anyone who says the guy was "enjoying the show differently" is an idiot. They were just screwing up the show for the people behind them, and they're probably a sociopath.
As for possible reasons he was on his phone;
Argument with partner/friend - possibility resulting in them not going - spilled into the show via text (should have left)
Planted security, already seen the show a dozen times, just waiting for something rowdy enough to require intervention
Music/event/venue reviewer (seen all kinds of reviewers who apparently have a 5 second memory insist on taking notes in-situ. Movie critics are the fucking worst about this.)
Work "emergency", fixing from phone
Could probably come up with more, but it would be along these lines.