What are the TWO possible reasons why an application failed to update on a user's phone?

An application is unlikely to update if there is not enough space on a smartphone or a weak Internet connection, regardless of the OS. It's good to have the phone plugged in while doing updates because allowing the power to go too low or the device to switch off during an update may result in problems. Mobile devices typically use Wi-Fi rather than Ethernet, so you'll need a strong WiFi signal.