It does sound quite bizarre and funny, but the woman suffered burns when it happen, leading her to being placed in a hospital. What has been determined is that it is the result of the rock being coated in phosphorus.
... By Friday, California environmental health officials had an answer, or at least part of one: two of the rocks were covered in phosphorus, an element that’s known for igniting into a fierce white flame when it’s exposed to air. Near as they can tell, as long as the rocks were wet with seawater, the phosphorus didn’t ignite, but after they’d dried out in the woman’s pockets over the course of the day, the phosphorus reacted explosively.
So...that can happen... Still, glad that was cleared up so as to prevent other wild stories of the exploding rocks of California...granted rocks can be caused to blow apart in other ways so it isn't the strangest of stories.
What isn't clear yet is where the phosphorus came from.
The most common way more reactive phosphorus is found is within the military, where it is used quite often in flares, though it has also been made use of as a deadly weapon. So there is so interest now about if some flares washed up on the beach, or if there is another, possibly natural, answer.
If this does come from old or more recent ordinance, it would be a wise thing to try and clean this stuff up to prevent further injuries.