Hello everyone. i have a dog he is half bull mastiff half staff?

my question is why dose my dog run away petrified from my nan. she is about seventy and quite normal . my dog is a big lad but very friendly loves everyone can anyone tell me if they have had the same sort of thing happen to them and hopefully a reason. she has met the dog on two occasions and both times it has run to the kitchen and hidden is she evil because she seems very nice my dog gose to any one but her am i part of the evil dynisty???????