[gavinjconway]

Getting back to the subject.

When this happens to seeds it is often that they are not planted deep enough or the compost is too loose and the seed coat is not held in the soil.

Brassicas all do it..  and so do most onion / leeks...

[azubah]

Aaah..that could be the answer.
If the seeds are sown deep enough the moist soil may help to soften and release the seed coat.
Seeds may be programmed so that some do better sown deep and some more shallow, thus ensuring survival of at least some of them..and perhaps allowing the others to come up later on if the soil is disturbed.