I had this with a man and woman walking three off lead dogs, one of whom was a staffie; have nothing against staffies, most of the ones I know are well trained, well behaved dogs.
However, they are often big, muscular dogs, as was the one that decided, out of nowhere, to pin on her back my, then, 10 year old, 5.7 kilo dog, whilst appearing to bit her neck.
Apparently, according to his owner, he was just playing. And yes, he probably was, if his neck biting had been real she would have been dead within seconds, or at least very badly injured.
Don't think my dog thought he was playing though, from the look of terror on her face.
After a lot of 'get him off' and such like from me, the dog was eventually, removed and they walked off saying he was only playing. I didn't respond.
Thankfully, she's a resilient little thing and after a bit of reassurance from me was happy to go on her way suffering no ill effects.
Had she been a more nervous dog I hate to think of the long term effects that could have had on confidence and well being.