My guess. Pure speculation, only based on the bits of information here.
These dogs were acting on guard instinct. The pit-bull terrier type was the leader and kept on with his natural endless attack which the Shepherd followed. Pack mentality.
Normal dogs do not do much except to try to break the behavior of the “victim”. Hence your “single bite”. Fighting dogs keep going, obsessively. PBTs are fighting dogs, and it was bred into them.
At the very least, she should’ve been acclimated to them much more than just meeting them once. Stupid to allow a virtual stranger to check on them when they seem to know the dogs go overboard.
Agree that the owners should have had the pet sitter interact with the dogs, take them for a walk, etc every day for a week or more to be safe-the poor woman was a total stranger to the dogs who were already psycho timebombs from being penned up all the time-stupid owners...