There are elements of mystery about this. Clearly they are determined to fix the bullpen once and for all. Rapada ? - let's see, and hope he's part of the answer.
I'm going to take an absolutely wild guess as to why Murton was sent down and not someone else like J. Jones (can Jones be sent down - can he refuse somehow?). He's being showcased. Other teams are wanting to see him play right now and he is just not going to get the at-bats with the Cubs. Maybe he should but... I'm guessing people are wanting to see him play to be sure he's OK etc. They've got to know he's a Major Leaguer and I'm still convinced he will be very good - not very likely as a Cub, unfortunately.
And let's not forget his amazingly weird inability to hit in Wrigley.
I suspect he has excellent trade value. The Cubs actually have quite a bit they could move and I will not be surprised if a lot of activity involving the Cubs occurs over the next 6 weeks. The Cubs just might be substantially different from the team the opened the season by the time of the "big push" (last 60 - 70 games). Let's see if they can make effective changes.
One effective change I think they have already basically made is a new keystone pair: the Kajun Keystone - Theriot & Fontenot.