So I don't know if it's as simple to suggest it was a pay-for-review scheme, but rather just its own thing.
Same thing happened on Amazon milk reviews: http://www.nytimes.com/2006/08/09/technology/09milk.html
The game was available on the store for seven months without getting much traction. The developer probably thought he would try some kind of pay-per-review/promotion service to get a few more downloads.
This got the game into the charts, but it only really began to take off because unlike many of its peers using pay-per-review the game actually had a hook that got people playing. In this case the pay-per-review was a tiny catalyst for vast growth.
Of course, this assumes the developer used some sort of paid promotion service.
Despite how rare it is to have a true grassroots viral hit, I think that's what happened in this case.