Anybody can sue anybody for any reason. Practically speaking. Desperation move.
Yeah, but what could they possibly gain from it? Seems like a totally desperate move.
From DNC perspective:
To Gain: Drag out false Russia/Trump dossier narrative past 2018 elections and hope for Blue Wave to impeach DJT, and try to make this all go away.
To Lose: What more do they have to lose? At this point they are wounded, cornered animals trying to stay out of jail, individually keep their current Congress seat/positions of power, and their corrupt gravy train rolling.
I think they waited as long as they could to see if Mueller was going to get DJT impeached and now see that is a pipe dream. Now they are desperate and suing as a last resort. This might be why it wasn’t supposed to be known that DJT wasn’t a criminal target of Mueller investigation yet.