What would make a dog foam at the mouth in hot weather with plenty of water to drink?

If the the dog is foaming at the mouth, but otherwise has enough water, it may be simply due to exertion. If the dog has been running and playing, they may foam at the mouth even if they have plenty of water. This is just the dog's natural air conditioning system at work. If the dog has not been playing hard, it may be a sign of injury or illness. It is not always rabies, many conditions both major and minor, can cause foaming at the mouth. This can also be a sign of anxiety. You will have to watch your dog for other signs of illness to rule it out. If in doubt, take your animal to the vet.