One major component: regional expansion. The launch of Apple Pay in China may have played the biggest role, but there was also a steady stream of expansions to key markets like Australia, Canada and swaths of Asia and Europe. Also, there were simply more people with Apple Pay-capable devices. You had to buy one of two high-end iPhones (the 6 and 6 Plus) to use Apple Pay throughout most of fiscal 2015, but the service was an option across all of Apple's phone lineup by the time the iPhone SE arrived in March of this year. That's also excluding those people who may have an iPhone 5 or 5s and are using an Apple Watch for their payments.
Whatever is involved, it's likely that Apple Pay will see continued growth for at least a while. The payment system reached both Japan (as of iOS 10.1) and Russia in October, and there's still room for both more countries as well as additional cards and stores in existing regions.
The question is whether or not Apple still has a lead in this fledgling industry. The company hasn't divulged its latest transaction numbers, you see. Samsung was quick to boast about having 100 million transactions for its own service in August, but the lack of context makes it difficult to say whether it's catching up (Apple is estimated to have racked up $10.9 billion in purchases in 2015) or trailing behind. About the only certainty is that Google's Android Pay will need to grow faster if it's going to latch on. It only just reached the UK in May, and card support isn't as broad as you get with its rivals.