Ghost Ryder

I think what MVcrash said is more likely a better solution.
Some dogs are more into the chase, & it sort of puts them into "the seek & destroy" mode.
Sometimes standing your ground works, but with overly aggressive dogs its not always the answer.
I'm quite surprised to find out it was a Great Pyrenees, they are search & reduce dogs, not guard/attack dogs. That's very strange behavior out of this breed.
Again I don't know how the dog was raised, or how the owner treats them.
Sounds like they reacted on nerves since its sounds like you didn't make eye contact. Eye contact can be perceived as a threat, or a challenge to some dogs.

On the other hand, these could be stray dogs, their tactics sound like pack mentality. One dogs keeps your attention, while another attacks. The attacking dog definitely knew to go for the back of your heel to immobilised you, or drive you away from their territory.