Why did he disappear??

So Thursday night my ex (of 2 years)came back into my life after 5 months of not talking. We had a very good night and he asked me out again and we talked about our problems and what we needed to work on to improve our relationship. Since our breakup I have met new guys and been hanging out with people, trying to have a social life and he seen that. He knew I was talking to other guys because I told him. He hates lies and I used to tell him little white lies, but this time I was determined to be honest with him, so he asked me who I was being intimate with and I named someone that he knew and he was disappointed. They aren't friends but they did go to school together and they see each other at parties, and they have short casual conversations and that's it. He thinks I slept with him on purpose, (not true) So after telling him that he said it was okay and that we were going to move forward. He was so happy to be with me and make things right. He even talked to my mom and told her how much he missed her, like he was just SO serious about making this work. He told me he knows I am the one for him, he's not never letting me go, etc... I stepped outside of my house and he walked to his car and I went back in the house. I noticed he hasn't came back in my house yet, so I looked outside and his car was gone. After calling and texting him a thousands of times, he responded and said "Sorry babe, Sara(His cousin's pregnant gf) "is about to have her baby, she's 5 centimeters, I will pick you up okay?" and I replied "Okay" He never came back and that was that. I called and texted him the next day, still no answer. So I was freaking out, and thought something happened to him, but my cousin called his house and guess what, he picked up, but hung up, we called again and his grandma said he was in the shower. That was that. He came into my life for 24 hours and bounced.

What was that all about? why did he waist his time to come back in my life for the 100th time and try to make things work but then leave? I'm thinking it's because I told him I slept with someone he knew. Could that be it? couldn't he still have the decency to text or call me and tell me that he changed his mind, or even if he was okay that night?

Why did he disappear??
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