It can be different things.
Some fights can be totally fake.
Sometimes people will provoke fights on purpose, so there will be drama to put on television. They might insult someone on purpose; or make personal attacks against someone; or yell at someone on purpose.
They might have something that is bugging them about another person, but then try to blow it up into World War III, deliberately.
These aren't exactly real, since they're trying to create drama...
But they aren't always exactly fake, either... if someone is mean to you on purpose, you're likely to get upset or angry. If they're insulting you (or harming your reputation) in front of a bunch of people, you still wouldn't like it.
Then there is real fighting. Some of that is the end result of people trying to start fights, like above. But some of it can come from other things happening in "real life," and more natural conflicts going on.
It's also condensed with editing. Just because 50% of the time on an episode is people fighting doesn't mean they spent 50% of the time that week yelling at each other.