Some candidates can't make it through an interview without being condescending to someone, making snide remarks, being arrogant, trying to start arguments about trivial things, or other negative behaviors. If they're doing this during the interview, you're only seeing the tip of the iceberg. It's going to be 100X worse when you have to deal with that person 5 days a week.
Other candidates behave well during interviews but have a history of causing social problems at other companies. It takes some work to uncover these (and, importantly, validate their veracity). Some of the most toxic people I ever worked with were very charming in interviews. They swung from company to company, leaving a trail of unhappy coworkers behind them. They could only get hired into new companies where nobody knew any of their past coworkers, because a simple reference check would reveal how difficult they were to work with.
Filtering this behavior out before someone joins the team is very important. Hiring a single socially toxic or subversive person into a team is like dropping a bomb on a healthy team dynamic. You may lose multiple good employees before you figure out what's going on and put together a case for firing the bad apple.