@BlakeRichardson is bang on the money with this!
For a point of reference a Wi-Fi download requires "a lot" less power so in both Android and iOS it will be the chosen method for any downloads. This is why when you have Wi-Fi enabled it will keep trying to find something.
And this is strengthened by the link provided, in that cellular bandwidth is known to additional 'cost' more financially. Some apps default to Wi-Fi only until told otherwise for this very reason.
This is also why iOS gives any app 20 seconds to shut off, but it is killed to prevent such download or drainage in the background.
Thank you,
Peter James