The ballot drop boxes in Washington and Oregon both have fire suppression systems that are designed to activate when the temperature inside reaches a certain point, coating ballots inside with a fire-suppressing powder.
For unknown reasons, the system failed to prevent the destruction of hundreds of ballots in Vancouver, just across the Columbia River from Portland.
I don’t think it’s an “unknown” reason; the system requires a fire to have already been lit in order to activate. It’s fire a suppression system, not fire prevention.
The person who wrote the story doesn’t know. It’s unknown to them.