Why does my ex ignore me? I don't pester him?

We work together, and at work he doesn't look at or speak to me unless he absolutely has to. He radiates tension and whenever we have to work side by side it's just awful. I try to be friendly with him, I haven't called or messaged him or hared him in any way to get back together. The breakup was mutual-ish, we weren't talking to or seeing much of each other outside of work, so I asked him if he wanted to break up, and he said "Yeah, I guess." So why the cold shoulder? He doesn't act this way toward any of his other ex girlfriends.