Weird. When I reject (X) a photo in Dev module, this and immediate next getting rejected. Same with (U) unflagging. Just got into this mode today, worked fine before. No multiple photos selected. Tried to restart LR. LR 5.3 Mac.
Probably some not obvious function I chose, or a bug in LR.
Ok, understood how to reproduce:
1) go to library module
2) Choose compare view (C)
3) Switch to Development module
4) Unselect photos Cmd+Shift+D
5) Now only current photo will be highlighted in the film strip. However if you press U/X etc it will act on two photos.