chuck b. mentions oak masting and provides the link to a Sac Bee article on masting (focusing on the Valley Oak, their locally native species.)
From the article:
Folsom city arborist Ken Menzer said the origins of the ample acorn crop stretch back to 2006.
"That spring was really wet, and because of that, in the fall the trees put out lots of buds for flowers," Menzer said. "Then, in 2007, we had a really dry spring, which is perfect for pollination. So in the fall of 2007 we had lots of acorns that were viable."
As for all those tiny trees sprouting in your lawn, it's too late to save them if you don't want to let them grow in place. Oak trees grow long taproots right away, making them hard to move.
"Right now, I hate to tell you, just mow them over," said Ray Tretheway, executive director of the Sacramento Tree Foundation. "When an oak seedling sprouts up, and the first two leaves come out, that taproot is already two feet into the ground. You can't dig them up and transplant them at all."
Of course the real mystery about masting is why it's synchronized across the entire oak habitat and across oak species. An earlier post right here pointed the finger at research that supports a critical window of spring time temperatures as a signal, and as you'll recall, spring 2007 (the end of 2006 winter) had several weeks of unseasonable cold.