Maybe she was caught in adultery and they ran out of stones??
Or “blasphemy” of their book.
Sometimes jihadists will kidnap and rape a woman, providing a basis for an adultery charge and also dishonoring her family. They offer her a choice: be stoned to death for adultery - the islamic way - or restore her family honor through martyrdom. I suspect the latter is the case.
That's actually a plausible theory. A woman who might otherwise be subject to "honor killing" could be offered a quicker and less painful way to die.