What metering to you have it on? My guess is it got darker in the frames because it metered on the white wall. If you look at the wall, it is almost overexposed in the first shot and as the shutter speed went up, the wall became exposed more properly

You could combat this by using centerweighted or spot metering, but, then again, it all depends on where your camera finds the "spot" to meter off of if you move.

I just do not use shutter priority because it ruins any creative effect I might decide on. If they are a bit dark, you can pull that up in PS. I do worry about highlights because once those are lost, they are gone.

I have run into problems with scenes like the german shepherd one. Those are the sort of things I use manual with. I often do so at horse shows because the meter will pick up the dark dense trees and overexpose the rails, or pick up the rails and underexpose everything else.