This tendency of talking behind someone's back negatively has become very common in today's society as a result of the following:
Idle mind is the workshop of the devil. Most idlers find themselves in discussing other people's negativities as they have nothing constructive to do.
People who have neglected their duties talk ill about those who are carrying their duties. They form their social groups of irresponsible people consoling themselves through such talks.
Some it's just their nature. They can't talk about good things in someone but only to dwell on the bad ones.
Once they lack confidence in approaching the person, they meet a group of cowardice people and talk of the other people's bad deeds in secrets.
Some people are not happy with other people progress in life. Instead they are looking for the downfalls and talk ill about you.