Many times what happens is she is already planning her exit before you know anything is going on. Since they are emotional and go on "feelings", They could start "talking" to a guy behind your back and although sometimes it just begins as talking... Things begin to escalate and she starts developing feelings for that guy. The fvcked up part is while she is beginning to feel something for a new guy, She will still put up the good girlfriend act and many times acting normal because if she begins fighting and arguing, She could give off a red flag that something is "off" about her behavior. Sometimes girls use the fight/arguing to provoke a break up that they want so they can go pursue someone else and so they dont look like the bad person initiating the breakup but usually its the first thing.
Once she's convinced that the other guy wants her and to be with her (or so she "feels" that way) Then she will begin to pull away fast while your feeling the heartache and wondering "What happened!". At that point she starts throwing a boat load of excuses as to why you and her cant work out, Why she doesnt love you anymore, why she needs a break...etc and while all this is going on, Us men do what we all have been guilty of doing... Chasing after her and trying to save the relationship before she is gone completely. She may seem extremely cold but what's happening is she will try to push you away so that you can be completely out of the picture if there is in fact a new guy. Sometimes though, she may still initiate contact with you while she has basically begun a new relationship OR is fvcking him already, Kind of like a false hope because at this point, she's with someone else and your hurting. In a nutshell, they basically cheat on you while you have no idea whats going on. Is this always the case? No. However, They all to some extent, do and say the SAME things.
Another thing, Since women are emotional and go by feelings, is the potentially horrible influence her friends are. Meaning, They will talk your girl into leaving the relationship by justifying Why she can do better and how she needs to "live life" and "be free" to have "fun" In other words....."You can do so much better than Mike, Your beautiful and young. You should be out having fun "meeting" new guys and enjoying "fvcking" i mean Enjoying life". While she may even know that some of her friends are little cvm dumpsters, All it takes is for the right things to be said at the right time and your girlfriend/wife will agree. Then venture off to break up with you and give you the "I need space" "I need to find myself" "I need time to think" "I need to take a break" "I need to focus on other things in my life". Its ALL bullsh!t because any woman who really loves her man will NOT! let him go. Women are amazing in how they rationalize anything even if its their fault,... They are emotional beings and therefore built to be lead by Men. Today, if a Man forbids his GF to do something, she may act out because feminism teaches that women are "empowered" which then leads to her going from c0ck to c0ck to c0ck.
All my ex GF's have contacted me and still do today except one, My recent ex i was with for 5 years and planning to Marry at the time. I loved her 100% but i will never forget how dirty she did me at the end. She knows how fvcked up the things she did that left me with anxiety and depression although im doing better now but i have received some phone call and hang ups that make me think it was her. When we had that big hurricane a while back, One night I got a blocked number call and when i said hello they stood on the phone for about 3 seconds before hanging up. I guess she wanted to know if i was ok without actually talking and seeing a phone number. she lied, a lot! about so many things when she seemed like the sweetest girl i had ever met... She left me to get engaged to another guy 2 months after the break up. I will probably never hear from her again because she knows that even 3 and half years after the breakup, I still carry that rage from what she did and will lash out.
All that mentioned above is from my experience and has happened to me bad twice, It happened just like that with my best friend also and I'd say 90% of my friends (which is a lot) It has happened to them too, amazingly in similar fashion with their GF's, Wives saying and doing almost the exact same things.