Realising you need to leave can be for so many reasons.
I think there’s a saying along the lines of ‘the things that are just irritating about your partner at the beginning of the relationship, will be the thing that you eventually split up over.
People change as they get older, or one person changes and the other doesn’t. Stresses can come into a relationship and it causes one or both to not behave at their best.
Often the arrival of children makes the woman see how bad her partner is at picking up the slack/how selfish.
In my case, because of my childhood I didn’t have good boundaries and didn’t feel I had the right to say when things felt wrong or I was unhappy. My ex was a bossy, pushy sort of person, but very caring and kind when he was getting his own way.
As years went by the bossiness became manipulative and controlling. I got more fed up and resentful. Eventually I started imposing boundaries (mainly allowing myself to say no to doing things I didn’t want to do).
After so many years of not getting pushback, my ex felt my behaviour was out of order, he got more pushy, I stonewalled, he got more angry, I disengaged.
So there you have it. His bossiness developed into outright verbal abuse when I stopped doing everything he wanted, because I had changed.
The original ‘red flag’ (bossy) was always there, but as the relationship floundered, our worst sides came to the fore. Eventually there was nothing worth staying for.