I don't think "common denominator" does imply fault. People often confuse cause with being at fault, and that's a mistake and stops people from seeing where they are making mistakes clearly.
I'd also say that toxic relationships aren't always about an abuser taking advantage of a victim. A lot of the time it is about two people with problems who make each others problems worse, or who are using each other in some way.
In my work I see a lot of people with serious drug addiction, and many toxic relationships. Some are women hooked up with dealers and pimps whoa re certainly abusers - in some cases the women know this and could leave but put up with it because of the access to drugs - it's the drugs which hold them there rather than a threat from the men who could replace her easily enough.
But there are also plenty of instances where they are both addicts, and they are better able to pursue their addiction together rather than alone. Some seem to hate each other, while others have real affection, but ultimately they are together because a non-addict would not put up with the lifestyle and drug seeking, and they take care of each other when they are using.
None of that makes for a healthy relationship, even apart from the drugs though, there is a lot of drama and even violence that comes out of the drugs and self-hatred involved in being an addict, and that gets focused into the relationship.
I think you can get similar, though usually less extreme, dynamics between couples who have other issues besides addiction, too.