That's evolution. The populations of the two snake species are experiencing selective pressure from the cane toad population. As that pressure continues, the morphology of the snake populations is changing. After 20-25 generations, that change is already noticeable.
I would say this is the first stage in the emergence of a new species. After many more generations of small headed snakes breeding, sooner or later they will either die, or the environment will change so that the big headed snakes are no longer being selected against, or they will be unable to breed with the big headed variety and a new species will emerge.
It's like that butterfly (or moth) that responded to bad air pollution in England and became darker so it couldn't be seen by predators. On a white tree, it was conspicuous, but on a tree in a polluted area it wasn't. The population became almost all dark. However, GB cleaned up its act and its air and the population has shifted back to the earlier white variety, since the trees are now cleaner. There wasn't enough time and selection for the dark variety to emerge as a new species.
If the Aussies start eating the toads, the big headed snake may reemerge as the dominant variety. If not it may disappear to be replaced by the small headed one. You can let that nutcase Steve Irwin decide if its a new species or not.