I bumped into my ex and he told me to "smile" then walked away... why would he say that?

So my boyfriend dumped me the day I moved back to uni (he is at uni here too) And he dumped me just because I couldn't talk on the phone as I was in the middle of watching a film, he told me it was over and to delete his number.

I tried calling a couple of times to reason with him but he just told me to leave him alone. A few days later he called me and asked how I was and was checking I hadn't done anything stupid, I asked him if he was still mad at me and he said "well I'm still your friend right?" i said I wasnt sure, we continued to talk on a civilised manner about general stuff not the relationship.

After that I didn't see him till today (couple of weeks later) I bumped into him I was outside smoking, he just walked past and said "smile" ....

I don't know if he was being awkwardly friendly or saying it to piss me off as he knows I hate it when people tell me to smile.... anyway I just kinda acknowledged him by nodding my head. I stupidly texted him after saying sorry if I seemed rude but its awkward seeing him atm.

Anyway why didn't he just ignore me or just say hi? why tell me to smile?