27" Cinema display needs MBP plugged in to work, odd

I have a 27 inch Cinema Display which I used with a c 2012 MacBook Pro (MBP)

I used the Magsafe 1 power supply on the "triple lead" on the Cinema Display (CD) to power my MBP

I have now bought a 2015 MBP

I need to buy a Magsafe 1 to Magsafe 2 adaptor

BUT.... in the meanwhile I have the MBP powered from its own mains lead. Even though the CD is still plugged into the mains , it switches off when I remove the mains power supply to the new MBP

Why would the CD not continue to use its own power supply?