There are several reasons for this.
Many children have a strong personality. They always feel their opinion should always be better than others. People with strong personalities have a deep-seated attitude of never letting anyone else outweigh them in terms of opinion and presentation. This inherent quality generates the urge to speak and sometimes counter what others have said.
For example, if the mom says, “it’s a beautiful clear sky”, an argumentative child is likely to reply back with, “it is not clear, it is somewhat grayish blue”.
Many other children simply want to speak no matter what. These kids are oblivious of the fact that they are triggering arguments with their statement. They do not wish to leave things unsaid. It’s just that they don’t want to keep it to themselves.
Children often imitate their elders. If a child sees his or her parents or elders arguing, it may pick up this habit. Child psychologists have also corroborated on this and this is why counselors always ask parents about their camaraderie before the child.