I think some people sabotage relationships for the same reason they throw video games. They have fears/suspicions that it’s going to not work out, and rather than be a victim they want to have some control over the outcome. A loss or failed relationship doesn’t hurt as bad if they caused it to end the way it did.

However in your case, it sounds more like a fear of something different. It’s a lot easier to keep things like they are, when a relationship gets too serious or life impacting it can be easy to be scared of the change, and instead subconsciously decide you want to keep things like they are.