You may never know what caused the problem but it's working now and that's the important thing. Thanks for letting us know.
Go into Settings>iTunes & App Store on your new phone and check to make sure that automatic downloads is turned off for the App Store. It's the common iTunes account that lets you download content onto more than one device, it matters not if those devices have different ID's and passwords.But now this boggles my mind even more. Now that I can download an app on my old phone it end up on my new phone - why? I have one iTunes account but these phone have different ID's I would think.
I bought a new iPhone 6 and kept my old iPhone 4s as it still works on wifi for listening to Pandora or what ever. So I went in and deleted lots of Apps to clean up the old phone and all is good except..... I see some Apps need updates. It seems there is no way to update the Apps on the old phone it just keeps spinning even when on a good wifi signal. So over time Apps like Pandora will need an update and I will not be able to do it. Is this phone basically going to be useless over time? Any way to download or update Apps on and old phone with no cellular connection?