It's obvious why he's doing it:
He's going through a particularly ugly divorce, with allegations surfacing that he was physically abusive towards his ex- wife and kids. He figures that leveling a ridiculous BS accusation against her will attract attention away from the abuse accusations against him, which he thinks will help ensure that he won't be taken to the cleaners quite as badly in the divorce settlement.
Just more proof of what a petty, heartless moron he is, and always has been.
Remember when he thought an online Q & A session on the DUmp would help his Senate campaign?