I don't know all the scientific facts behind mood swings and pcos but they are related. Here is how my gyno has explained it to me. Our bodies are constantly releasing hormones, estrogen and testosterone. When we cycle our body flushes the hormones. So for us PCOSers that don't cycle every month, our hormones are building up to outrageous levels. Estrogen is the number one thing behind our mood swings that's why woman have more mood swings than men, we have more estrogen. Because I am generally a very emotional person anyway, my gyno fears when I don't cycle atleast every 2 months. I think my moods got him a little freaked when I went through some serious mood swings at my last visit. I could be way off on this too but that's how I understand it to be.
Hope I helped!