This phenomena is not unique among some species of toads. It is also been known with some species of snakes and lizards as well whose offspring hatch inside of them before being born.
Likewise, the offspring of these toads probably undergo their entire development stage inside of their mothers instead of outside of their mother’s wombs as is the case with most other frogs and toads and likely due to the offspring of this particular species undergoing an accelerated development.
https://www.livescience.com/animals/toads/newly-discovered-toads-skip-the-tadpole-stage-and-give-birth-to-live-toadlets