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.
The explorer is being assisted by a god, who is trying to save him from another god, who is trying to kill him with the never-ending templeSo there are two gods. One god owns the idol and temple, and the demon monkeys are his servants. This god is trying to stop the explorer from stealing the idol by manipulating the geometry of the temple and sending his minions after them. The other god is interfering by giving the explorer temporary superpowers at random intervals, and providing magic gold with which to lift his/her spirits and help them more effectively use the powers for longer periods of time. Every time the explorer dies, the second god resurrects them. These gods are banging heads over their disagreement regarding the fate of the explorer, and this will go on either a) forever or b) until the gods resolve their dispute and end the explorer's run. Meanwhile, the explorer is trying to call for someone from civilization to come and rescue them, but the maliciously minded god is using his powers to jam the satellite signals. So that isn't happening anytime soon either.