I think this phenomenon is not uncommon. It happened to me growing Rosa rugosa from seed I collected (but I had saved the container) and today I identified something that did not come up from a 2012 re-seeder, Rudbeckia triloba. I guess the conditions must be just right for natives to germinate. Only Mother Nature knows.

I had read somewhere about seeds somehow being genetically programmed so that they all won't germinate the first year. If I recall, some would take up to 7 years to germinate. The scientists thought it had to do with species survival. If there was a drought or flood or some other extreme weather event, and all the seedlings one year were wiped out, there would still be seeds available in nature to germinate the following year so that the species would continue to survive.