Flash goes off with ample light conditions

Yesterday I was using my D5100 outside with ample light ambience, most of the time everything went fine. Every so often though, for no apparent reason the flash would pop open and also go off. I went over all my settings and could not find anything that would provoke that. Does that happen to someone else too? Or, second question, any idea as to why this would happen?

I'm guessing you were shooting in auto mode. Try switching to the mode with the lightning bolt with the line through it (no flash).

Why did this happen?, I would guess the camera wanted to use "fill flash" for the shot you were trying to take, or you were in macro mode. If I remember right macro likes to use the flash all the time.

Welcome..Try shooting in aperture priority...