Thank you for asking, several reasons actually;
First each of us were going through a MLC at the same time.

Our biggest issue was, he has a close friend (she was both of our friend) told him I wanted to have an affair (she was having an affair). She and I obviously had a falling out because of this betrayal, which he said he saw as simple little issue except at the time he confronted me with her statement. This simple statement caused many issues and fights.

This "friend" would drag him into our disagreements by copying and pasting letters, emails etc of my efforts to resolve these issues to keep peace in my house. I even begged her to resolve the issues she had with me with me so that he wasn't stressed about it but she could not help herself I guess.

She also was way too involved in our relationship and made her opinions known about how she felt about me and our marriage. She would constantly manipulate facts, caused confusion. He defended her actions, comments etc. I suddenly felt very alone I didn't have his support as a husband as he didn't have my back and I just couldn't fight it anymore. I felt he didn't care what my feelings were (losing a personal friend, actually 2 him and her, she drug nearly all of our friends into this falling out. I tried to walk away, but just when things would be calm she would find some minor to keep the drama going. That said I am sure I was constantly reacting to panic and stress over the situation and certainly didn't handle it well

Other reasons, he had been pretty checked out for sometime. He didn't feel I contributed enough and frequently told me he felt everything was my fault. He couldn't "remember" details of things like asking me for a divorce at Christmas, all he could remember was I left at Christmas not why or that I had been texting and calling to try and work it out. Many examples of his "forgetting details". Too many of his friends blatent disrespect like screaming horrible names in my face and him not defending me, same person came to my house shoved a camera in my face and wished I would die, he blamed me. I tried to tell him that if they don't respect me they don't respect him either.
Some of his "friends" created lots and lots of lies and rumors that he knew the truth but believed them because they were saying it. After a while I got tired of being the target for everything that was wrong and walking on eggshells. I became very despondent really didn't care about anything because I was doing it all wrong.
I asked him to go to counseling and he made excuses so I started going and she encouraged me to leave.