My PHP application went down for a few hours with 502 bad gateway. In the nginx error log all I see is:

2013/11/14 10:02:16 [error] 1466#0: *57964 recv() failed (104: Connection reset by peer) while reading response header from upstream

I fixed it by restarting php-fpm. However, what caused this? I don't see anything interesting logged in the php-fpm or php logs. No indication of why happened, and why simply restarting php-fpm fixed the issue. I thought that php-fpm auto-restarted processes if they fail?

Any insights and place to determine the root cause? Thanks.