I’ve never heard of zoas, palys, or any similar polyps (ones that grow from a purple matt) releasing voluntarily. However, I have seen them removed by hermits, snails, and fish... and then blown around the tank. So maybe it’s possible, but I’m in doubt.
That said; leathers, anemones/mushrooms, Xenia, and similar corals are definitely able to release and float around.
In short, I suspect that they either weren’t originally sufficiently attached, or that their dislodgment was assisted by another organism.