Why does Apple Pay Passbook take over your iPhone 6 when you want to use LoopPay?
Apple Pay is designed to activate when near a compatible terminal reader. Your iPhone automatically senses it and switches to the Passbook app for Apple Pay, and displays the recently used payment cards. That's why the LoopPay app disappears.
(By the way, this is not a problem for a majority of merchant terminals in the US which aren't yet set up for Apple Pay.)
If you want to use LoopPay to make your payment
You can use Button Pay and your default card on your LoopPay device to pay....... Or you can also reset the default card.
To change your default card - move your iPhone 6 away from the terminal and follow these steps.
Tap to open the Payment Card you wish to set as your default.
Choose Set as Default.
Now you can use Button Pay on your LoopPay device to make the payment.