i think it's because they have very serious looks on their faces and they've got muscles, as my DH says. When i look at pics of cassius i have one that bascially shows his, "I could kill you if I wanted to" facial expression. But then, right next to it, I have one where he is lying down on his frist trip on a boat and his face says, "Eek! It's moving, what do I do?!?" and he looks like a scaredy cat!
When I see pp that are apprehensive of cassius, I try to point out his nubby- waggin away, it usually communicates that he is not satan and is not mean.