The explorer is already cursed to deathIt happened when he/she picked up the idol. I know it's kind of obvious, but it's not outright stated in game either.
The explorer is already deadHe/She is in hell, and is cursed to eternally run an endless path in a non-Euclidean environment with no exit.
The game takes place in the Skullmonkeys universeAs you're chased by Skullmonkeys (S)He successfully escaped the monkeys, idol in hand. But is now plagued by nightmares of what could've happened had (s)he failed.