There is no way to know why and I wouldn't worry about it too much. What it does say to you is that your friendship wasn't important enough for him to keep putting energy into it, therefore there is no reason for you to put any more energy into worrying about why.

keep in mind if you're a female and he's a male, he may have met someone romantically and could have had to cut off ties with other females for this reason. Not that it's right or a good thing but some people are quite jealous of past and current relationships and are inherently suspicious of their so being in relationships with the opposite sex at all. That would explain the untagging of himself and the non communication in this context. It's not an answer but a possible reason. But it could really be any number of things from him being just not as much of a nice guy as you thought to what I just mentioned.