Question

Why did sherif's participants change their estimates when they had to call out their answers in the presence of other people?

Answer 1

The participant's changed their answers to conform to the group standard

Every individuals have the inner fear of being outcasted by the other members of a social group.
To avoid this, most people develop the tendency to conform to a group's opinion even if they personally believed in different things.
One of the most known studies that first introduced this tendency to the world is called  Asch conformity experiments