He seems to be one of those people who’s incredibly rude to service workers. He was banned from a restaurant in NYC and the owner went off on his behavior there:

https://people.com/food/james-corden-banned-from-popular-nyc-restaurant-as-owner-calls-him-most-abusive-customer/

He's unfunny, cringe in how disingenuous his attempts to be so mass appeal are, he's a dickhead to people he deems to be beneath him like service staff in restaurants, and he ruined The Late Late Show after Craig Ferguson retired.

Put simply he is a massive asshole off screen, and is often not much better on screen

First, it's trendy. Second, he is kind of an asshole, even on TV.

Because he was in one show in the UK that was popular and it went to his massive head. A lot of UK actors realize that if they want to do well in hollywood there is this particular level of behavior that is expected of them because they're British (Of course that's a stereotype, but that's Hollywood). But he's too dim to realize that.

People actually liked Alan Rickman which is why he was in everything.

No talent ass clowns tend to draw ire. /shrug