I think that it's interesting that the wiki article refers to it as ritual killing of the elderly. While it probably is part of it, the wording doesn't necessarily strike me as something done purely out of necessity.

Maybe in some areas hard times came and the elderly volunteered to die for the good of their village or whatever, recognizing that they would be a burden and never contribute meaningful labor again, and then it became a sort of expectation/ritual? I'm going to try to dig a little more into it. It's certainly piqued my morbid curiosity.