The delegation was given to Musing by Mr Delegator which is ab account belonging to Steemit Inc.
This delegation is to enable musing.io to grow and get on their feet. They make the rewards from curations on each vote they hand out. I don't know if they have a time limit or more will be added as it becomes more popular.
I think but not sure but SMT's for musing will be handed out or earned against their delegation.