Well clearly something has happened. Either they've entirely parted ways with Nintendo, or they've decided to no longer limit themselves to just making Nintendo games, and are stepping out on what had appeared to have been an exclusive relationship.
I wish them well, but I'm far less interested in a Nascar game than I would have been of a new Excite game from them. I don't begrudge them from finding work from wherever makes sense for them to go looking, but I am a little disappointed, both from the perspective of this particular studios output and from what it could possibly suggest about Nintendo's ongoing relationships with the Western studios that they have long term relationships with, but no actual ownership over.
Monster Games is not a large developer. I'd say they're a very small one (with a website from 1995). I don't know what their involvement is with this, but I cannot imagine it being the grunt work of development.