why did my video on the S9+ suddenly start doing a slight dim and bright auto adjustment

[daddycools]

I was taking video of an outdoor event and everything was fine. I'm using an Smov gimble unit. Then the last few interview videos the whole screen seemed to be automatically adjusting the brightness up and down constantly throughout? What would do that? I had a hard time seeing the screen is there something I may have pressed to get it to do that without knowing it? IT was really sunny.