Gunicorn workers restart by connection reset
I mis-diagnosed the problem. It seems that both Firefox and Chrome perform certain optimizations when loading the same page address multiple times. I thought the server was becoming irresponsive, but in fact there were no requests generated to serve.
It seems to me that my gunicorn workers are restarted each time there is connection reset by a browser (e.g. by reloading a page while a request is still progress or as a result of connectivity problems).
This doesn't seem to be a sensible behaviour. Effectively I can bring down all the workers just by refreshing a page in a browser a few times.
Questions:

What are the possible causes for a gunicorn worker restart?
What would be the right way to debug this behaviour?

I'm using Pyramid 1.4, Gunicorn (tried eventlet, gevent and sync workers - all demonstrate the same behaviour). The server runs behind nginx.