Well you learn something every day (thank goodness for brain stimulation!).
It seems it is called "seed shattering" and is common to Cupheas. It's a form of seed release aimed at widest possible dispersal by wind or water, and not necessarily when all the seed is ripe.
Apparently it is also a trait in wild rices which scientists have genetically modified over the years so that mature grains (seeds) are retained instead.
Remains to be seen whether my collection of seed germinates! It's certainly a pretty plant.