Nikon changes manual exposure on me

Ok, I'll admit it I'm a Nikon noobie. After 40 years of shooting Canon the new D800e lured me over to the dark side.

I was just doing some studio product shots. I had the camera set to manual 125th @ F9. But every time I took a shot the shutter speed changed. Sometimes to 1/500, somtimes to 1/60.

I'm I doing something with Nikon I am not aware of? Is this normal?

TIA