Why does he disappear mid conversation?

So this guy and I used to talk allll the time. Then things kind of fell off between us but we still keep in contact for the most part. Well anyways, it doesn't matter if he initially contacts me or if I contact him, he always disappears in the middle of the conversation. Its annoying because he was never like this before and I know he can hold a conversation because we used to all the time. Now it just seems like he'll get back to me if and when ever he feels like it even if he contacts me first. I guess it wouldn't be so annoying if he originally started off like this. When we are talking, its great and we're both laughing... then a few texts later he disappears. But I just wonder what happened? Mind you, he used to talk all the time regardless of what time of day it was, if he was working or not, etc... Any ideas?
Why does he disappear mid conversation?
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