Q: Why did Apple Pay pop up on my phone?

Don't know. This has never happened to me.

George

You may have accidentally double-tapped the Home button while the phone was locked. Also, there have been odd reports of the Home button on some phones being over-sensitive - the reports I've seen have been about Siri popping up with even a short press/tap on the button. There may be a variant of that which caused this odd behaviour on your phone.

It's not something that is triggered externally

if you out your phone near nfc of any contactless card payment terminals, apple pay will pop up by itself. I tried it on the bus where we use contactless cards, it did activate on its own. So there you go

You may have accidentally double clicked the home button when you grabbed the iphone from your pocket. That always triggers apple pay.

Thanks for participating in the ASC. However, that is a duplicate of the same answer provided for in August of 2015, when this thread was started, one year ago.