I suppose if his gf/ow is a 'better' woman than me, then he might not be abusive to her....I would gently discourage this thinking, FolkGirl: it puts you very, very close to blaming yourself.
Abuse covers a spectrum of bullying behaviours. As well as more obvious things such as cruelty, and name calling, there are subtler behaviours such as discounting, disrespect, disdain, countering, eye rolling, unfunny jokes, undermining. And yes, the bad mood 'out of nowhere' that 'must be stress'- that the victim finds herself accommodating and buffering almost without thinking.
These subtler behaviours soon train the victim to behave how the abuser wishes.
Even earlier than that are the clever manipulations - 'You're so thoughtful. You're different from anyone I've ever met. My ex never made a fuss on my birthday/insert required behaviour here...'
There is no escaping that inherent, pathological selfishness, and desperate need for control. It shows itself in one form or another, whatever the partner does, though it can take years before the target realises what's going on.