What does my friend's actions mean?

To boil this down shortly I once had a really good friend in college but he always "busy" with things. I wanted a close friendship but he never had time...there was always other things to do and other people to hang out with. Eventually I read this as he didn't have time for "me" specifically and I started distancing. Wasn't hard because I was always the one who texted first etc. when I left all the contact up to him we went from hanging out weekly to hanging out once a semester. So fast forward a couple years...

After nearly two years post-graduation, (and his marriage whichI wasn't invited to but many others were) he messaged me a day or two before Easter...asking about my dog by name! (Weird) We exchanged some texts, and he asked about my life and my plans for law school, I answered and eventually, he quit responding after I answered one of his questions. No drama, he just never responded...Like he usually did.

About a month later (roughly two weeks ago) I checked my emails one day, like I do everyday, and I see he has responded to an email I sent to him over two years ago. Just a paper I wrote I thought he would find interesting. In the email he said "thanks for sending this, I didn't see it at the time. Must have been busy. Hope you're doing well."

What's going on? Why is he doing these odd things after two years of not speaking? Is he just checking on me, is something bothering him, or is this just some kind of gag? Should I respond?