It's actually completely *predictable*. If you have a tool selected other than a transformation tool, just click back to a transformation tool to be able to right click select UV's again.
I get this from time to time if I have a tool selected that the UV editor doesn't like (pretty much anything that isn't one fo the three basic transform tools) or sometimes when I've selected UVs on the model rather than in the UV editing window. Like most of Maya, it's all a little unpredictable, but it shouldn't be too…