I think what MAN are trying to avoid here is the situation where MON/KPMG continue to hold the slots looking for someone to buy them, but no-one is willing to pay for MAN slots and some form of stalemate is reached. The main objective for MAN must be to make sure they have every opportunity of being used for S18, regardless of who gets them.
Now they are back in the slot pool this means that airlines can apply for them now, and can factor them into any remaining S18 planning wiggle-room fleet and recruitment wise. Even a couple of weeks from now, this may be more difficult as airlines may need to source wet leases, 2nd hand aircraft or redeploy from other bases in order to utilise these slots - that takes time and if not done promptly could mean needing to cancel flights elsewhere in their network which are already on sale.
Take U2 for example, if they want to grow their MAN presence (I don't know if they do) they might need to forego growth elsewhere. I don't think all of their network-wide new routes or base expansion is yet on sale, so they still have the option to do this. But by mid-December they'll have more on sale, which makes it a lot harder for them redeploy and expand MAN.
Similarly in the case of LS, they might need to source more 2nd hand 738s or 330s.
So in summary, I think this is a sensible move from MAN to maximise their 2018 passenger throughput potential, and may or may not be based on intel from their airline customers around their plans.