That's because you are running Odoo server without workers and that way Odoo could be starving more resources than needed and also using it in that way could get freeze at any moment. You should specify workers options in you Odoo config file to limit the resources. 

I suggest you to read this blog post for the details:

http://www.mindissoftware.com/2014/11/23/Odoo-Start-Process/

I quote: "In short, there are four threads running now: the main thread, two cron thread and one WSGI application thread.".