Because when you talk in a group and all the attention is focused on you, you feel like you're put on the spot and need to perform. It's called performance anxiety. It happens in a lot of other areas of life too like a musician playing an instrument. They might play calmly at home but when they're in front of an audience they get nervous. The best way to get over it is to practice. Just get used to talking in front of people, it gets easier as you gain more experience :)