My ex and I starting to awkwardly talk again…?

Basically, he ended things about 3 months ago, saying that "we couldn't work and we weren't meant to be". Through all these months, we never spoke a word to each other but we would definitely look at each other and he would stare at me a lot sometimes and he mentioned me to my friend once. But besides that, I sort of joked with him two days ago for something clumsy I had witnessed him doing and he reacted kinda awkwardly and seemed really nervous. So it was like a two sentence exchange, and then today, he was waiting in the lunch line right behind me and he kinda quietly and awkwardly said "Hi ____ (name)". I don't know… I just noticed that when I tried to talk to him in these past months, he could never look at me in the eyes. And he just reacted really weirdly and always quickly cut short… why is that?