In Order to Meet You...
It's not that Nidorina doesn't have better things to do than await Nidorino's return (or in this case, meet him there), but Nidorino is her favorite person in the world to be with. The heart wants what it wants, man. This is a little OOC in that she'd never leave her children unattended, and if she left them with Nidoran and Nidoran, this would still have been planned earlier and not last minute.