They were distance markers, they're supposed to stand on the outside of the edge of the 'wedge' that the javelin area is marked in, but as the woman writes, as the javalin came down it veered to the right and sammy wasn't paying attention.

Clearly if anyone is to blame, it's either sammy herself for not paying attention while doing a job with an amount of danger, or the school for not making sure that the markers were staying aware of their surroundings.

But in the end it's an accident and these things happen - thankfully rarely.