Doggy drama at the cafe!

[Phteven]

Thank-you everyone for your replies and advice.
Bella isn't an old dog - the vet geusstimated her age at 18 months when we got her so I think she must be about 4 or 5 now. So it's not the onset of senility.
The man couldn't have trodden on her, he was too far away when the snarling commenced. Since Bella's lead is 90cm and she didn't reach the end of it I don't think she came close to him but since he was approaching from behind my shoulder he may have been closer than I realised. I'm not sure that she would have bitten him if push came to shove. When people come to her garden she tells them that she's going to rip their throats out but if they ignore her and push the gate open she retreats to her kennel or the other side of the garden and swears at them from a safe distance.
My BIL has had her to stay and he breeds and trains dogs and he says she's a kindly dog, our vet used to have her to stay when we went away and he likes her and when she goes to the doggy hotel they are always happy to see her. Until today no-one has ever suggested that she might be anti-social or called her a "bloody little thing" - I've always had nice feedback. Even the feral with other dogs issue isn't an issue if I'm not on the end of her lead. She socialises happily on sleep-overs.
I guess it could be something about him that was triggering. She came from the pound and I have no idea about her history other than she was found wandering with an equally cute sibling who was adopted before her. My Dad is about the same age and size as the man today and I don't remember anything odd about her reaction to him which makes me think that she was normal (my cat on the other hand loved him and step. I can only assume she used to live with someone elderly).

In short..... I'm still mystified and I don't feel comfortable following our normal morning routine tomorrow.