I think there are two things at work here:

-An expectation that because a scientist is incredibly good at one thing, they must be good at another, entirely different type of thing. This seems to uniquely apply to scientists.

We don't judge a phenomenal football player for not being an incredible archer, but then we do judge a Nobel prize-winning physicist for believing in astrology.

-I think this can sometimes be two sides of the same coin: The same divergent thinking that helps you make revolutionary discoveries (you're unwilling to take the status quo for granted) also makes you explore "strange" ideas.