What causes a person to be afraid of talking to other people?

I never talked to a teacher until Grade 4. I’m not really sure why. I had this incredible fear of teachers. Now, I love talking to teachers and get along with the ones I know at the school I go to really well. I’m just confused why I was afraid of them in the first place, so much to the point that I spent so much of my early education avoiding talking to them. What could have caused this? Also, do any of you have similar experiences?