It is immigration that explains the vast majority of the increase.
Who is it that immigrates? The poorest of the poor, the desperate ones that would be dead from starvation or execution by gangs/guerillas within 5 years anyway. These were not the ones being converted in their home countries - when you have to work 14-18 hours a day, 7 days a week just to feed your family, there is no time to "study", attend meetings, go out in field servce, etc.
So, they go to their new country. They can work a "mere" 8-12 hours a day and earn a year's salary (by their country of origin's standards) in a few weeks. For the first time ever in their lives, they have free time on the weekends, maybe even a few free weeknights.
Now here come Joe and Suzy Need-Greater offering them magazines in their own language - the first printed things in their own language they've seen since arriving. Joe and Suzy offer to take them to this Sunday's meeting, where they are "love-bombed" into an instant community of dozens of people exactly like themselves.
It's a highly appealing offer, and quite successful. It is why the WTS is emphasizing over and over for publishers to learn another language and change to a foreign-language congregation. Witnessing to recent immigrants is the one and only perfect setup for the Witness model of door-to-door preaching.
So long as there are economic imbalances in the world and lax enforcement of immigration policies, expect to see similar increases in the years to come.
Of course, there will soon be new, different problems. All these recent converts lack "spiritual maturity" so there will not be enough elders to govern the flock. Many are "illegal immigrants", and so can't serve as elders or MS. Watch for a spate of "Brothers, you should be reaching out" articles over the next 5 years.
But for now, the WTS will crow contentedly about "Jah's rich blessings" and the rest of their usual claptrap.