2048 worker_connections are not enough while connecting to upstream

[John Moore]

I'm using nginx as a remote proxy server in front of some Tomcat 
instances, with pretty high traffic. Suddenly yesterday afternoon we 
started getting the error in the message subject, one I'd never seen 
before, and it's occurred tens of thousands of times since. Now, I've 
upped the worker_connections count to a much higher figure and the 
problem has gone away for now. But I'm concerned that it might be 
indicative of something else - some kind of DOS attack, maybe? What does 
this signify and why might we suddenly start getting it? Thanks.

John