All this is hypothetical and assuming what the story says is true ( always a shaky assumption), but I think the dementia is the crucial point.
I suppose it could be dirty money, but in that case I wouldn’t want to touch it with a 10 foot poll. Other than that I can’t think of a reason, except organic brain damage, someone would do such a thing.
“I suppose it could be dirty money”
Well if it wasn’t dirty before they buried it in the garden... *rimshot*