What happened in the space of a week that caused my ex to have a seemingly split personality?

I went out with this guy B for 3 weeks; complete honeymoon stage. Then, he told his ex about us. Known each other 15 years, they dated 2 years ago for four months, until she cheated, lied, and then told B that she didn't love him. They stayed friends. When he told her ab us, she flipped **** and wanted him... show more I went out with this guy B for 3 weeks; complete honeymoon stage. Then, he told his ex about us. Known each other 15 years, they dated 2 years ago for four months, until she cheated, lied, and then told B that she didn't love him. They stayed friends. When he told her ab us, she flipped **** and wanted him back. He and I agreed it wouldn't be fair to string me along while he still had feelings for her. He talked about getting in a relationship again with her. He asked, "I have a horrible hypothetical..if I do choose my ex, would you still want to talk to me, maybe be friends?" I told him I did and I proposed hanging out on the 21st when he came to my city for business. He said, "That's a fantastic answer."
The 21st, he got out of the car, hugged me for a few seconds and said it was great to see me. He said he had been looking forward to hanging out w/me. He was still messed up ab the situation and asking other people for advice. He told me I was the "perfect woman." He still didn't trust his ex, he had hung out w/her but she hadn't seem to have changed. He told me he wasn't really considering a relationship w/her. I said, "Well, you could have decided you didn't want either of us, which is a valid decision", and he said urgently, "But it doesn't mean that at all!" He said he felt awful for having made me upset, that I didn't deserve any of it. He told me he had really just opted out of deciding anything and he wasn't happy about it, and needed time to figure out what he wanted. I was confused, he said, 'I still like you if thats what you're confused about." I brought up being friends and he got weird ab it and said quietly, "I have no problem being your friend." He added later he hoped being friends wouldn't confuse me. When we chose places to sit down, he always steered us towards tiny couches where we had to sit fairly close. I told him I'd leave the details of us being friends up to him, and he randomly said, "Well, I'm coming in again next thursday if you want to hang out." All in all very sweet/attentive.
Next thursday, completely different. This time, he didn't get out of the car, he just leaned over and unlocked the passenger door. He was still friendly/nice to me, chatty (nothing personal tho). But this time, he made it a point to not sit next to me ever...we went bowling and he sat in the seat back to back with mine, effectively sitting in another lane.He described us bowling as a "way to pass the time". I asked how he and the ex were doing, he stammered out, "We're good, we're getting along I guess," no other info. I teased him playfully ab a restaurant he had stalled on visiting for a year that had since gone out of business, and he told me to shut up. He didn't censor himself..in the car at crosswalk, he commented on a guy walking across the road with an irritated, "Walk across the road, fatty." He brought me back to my building, said, "I guess I should at least walk you to your door." At the door, he gave me an awkward hug, said, "It was nice seeing you," and left. No word since, obviously. I haven't contacted him and have no plans to.
WTF happened in the space of a week? What exactly made him go from seeming like he still liked me as a person to seeming like he hated me a week after? Some people theorize that he's back now w/the ex and was trying to be a loyal bf, but why the personality transplant?
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