Many northern species go through quasi-periodic irruptions.

I just went to the historical results page at:
http://www.audubon.org/bird/cbc/index.html
and looked at the yearly counts for Blue Jays, Clark's
Nutcracker, Red-breasted Nuthatch and Mountain Chickadee.
All fours species show a significant number spike in the winter
of 1989-90. Blue Jays show an incredible spike that season while
the other three show equal or great spikes in other years.

So, while this does answer the "why", it does provide a temporal
perspective. Irruptions happen.

--
Mike Patterson
Astoria, OR
celata at pacifier.com

Common species are more common than rare species
--- Dennis Paulson in
_Shorebirds of the PNW_