The Night Mother became an idiot is what happened. She's the link between the Dark Brotherhood and Sithis, and yet she refused to speak to any member for too long. The DB could hardly continue to function the way it had, and with its other woes, it was only natural it'd start to fall apart. What's really surprising is the lack of a competing faction muscling them out.
It's not that the night Mother became an "idiot". Never speak in such ways about our Unholy Matron!

There was simply no one deserving/qualified for the job, thus she went quiet. For more details, see below.
But under Astrid's rule, they had abandoned the Old Ways, and the Night Mother likely felt they were unfit to hear her voice if they couldn't follow the Five Tenets. The Dragonborn, being both insanely powerful and new the scene, seemed like the best fit for a new Listener to her I guess.
This.
It's because no one was worthy that no new Listener was chosen. And when that S'wit Astrid chose to abandon the Five Tenets for good, well that just sealed that there would be no more Listener for quite some time.
No one knows the thoughts of Sithis. A new Listener is chosen based upon what He sees in their future, and their ability to run the Dark Brotherhood in the way that the Dread father desires. He then relays such information to the Night Mother, and she influences events to play out in a certain way so that said individual
becomes the Listener.
In this way, the Dark Brotherhood gets the best possible candidate for the job.