…does this mean the player can choose to discard a Keeper with the intention of moving the Creeper OR the Creeper may only be moved if other cards caused the player to discard a Keeper.

A: It’s voluntary. You have to choose to sacrifice one of your Keepers in order to gain the benefit of getting to move a Creeper. You don’t get the benefit if it’s some other random thing that makes you lose a Keeper.

On the plus side, this means you can do it whenever you choose to. On the minus, it’s not some sort of consolation for having to lose a Keeper involuntarily. “I lost a Keeper, but at least I get to move this Creeper away from me.” Nope, doesn’t work that way.

See also: When a Keeper/Creeper (or Rule) says I can do something if I discard or move something, can I do that as a Free Action or…
And also: