I've found fossilised sea urchins before but was possible to see lines where it was 'divided'.Wonder if yours are formed like the Moeraki boulders in New Zealand?Something to do with layers of sand/sediment shifting over millennia,they slowly rise up out of the sand.Some Maori legend about a giant who threw them all there I think.
Being a geologist, but not a local (not even close, I'm in California), nor familiar with the location, the violent ejection theory is probably unlikely. Violent ejecta are usually associated with a lot of gases, which also typically results in quick cooling (quenching). Quick cooling is how you retard crystal growth and create glass (vitreous rocks like obsidian and the like). These are probably sedimentary or metamorphic rocks, but just my two cents. Very cool find! Being a rock nerd, "I dig it!"