It looks like this feature is controlled by the Group By option. In Audio, it works as expected because you don't see the Grouped By option. Audio is automatically Grouped By -> Album, and Album thumbnails are displayed on top each other. For example, in Genre view, if you hover over a "stack" of Albums for a particular Genre, it plays everything in that Genre. If you hover over a single Album, it plays the entire Album.
In Images and Video, files are displayed as individual files. If there is no grouping (default is None), it will play everything in that view (ie the whole "Group"). Now if you change the grouping (little down arrow on the tab -> Group By) you can control what it plays. For example, in my TV Shows view, I Group By -> Series, so it plays the whole series. If I Group By -> Name, it will play just that file.
I couldn't find a Wiki entry on it, but this is the way it appears to work for me.
Indeed to me it seems wrong as well. It happens with audio files too (if I change the List Style from Details to Thumbnails, clicking Play adds all currently visible files to Playing Now)
I don't understand this comment. There are no hover options in List Style -> Details view.