I think they have simply not allowed a sufficient amount of time to make the decisions by the 15th. Or they do not have the resources do get it done in time (lack of resources (to do interviews but a lack of resources nevertheless) was something that the guy heading the MCL programme complained about). I don't think they have such a thing as unofficial waiting lists.
It would take them half an hour to send out an e-mail to everybody apologising for and explaining the delay. It's just insane that they have not done that already. Burying their heads in the sand in relation to the deadline is unprofessional.
I think the delay was mostly caused by the academic officer that heads the degree committee changing during this academic year.
