This type of foaming at mouth is quite common and is normally a result of the dog’s state of mind rather than what they are playing with. Some dogs show this phenomenon when excited, scared, or stressed. Others show it as a sign of heat stroke from lots of running around. In most cases it disappears when the strong inducing element stops and is nothing to be concerned about. If however your dog shows these symptoms in one particular field or location then it could be something more obscure, such as a reaction to a specific vegetation or smell.