This is kind of hard to explain, bare with me.
Using $mdToast, if you click one of the buttons to display a toast, rapidly enough so the toast does not disappear between clicks (and multiple toasts are created within 1 digest cycle), it will increase the watcher count considerably but will not remove all of the watchers for all of the toasts.
$mdToast only decreases the watchers for the last toast created within a digest cycle.
Demo: http://codepen.io/anon/pen/avLPqP