A girl once broke up with me for not contacting her or replying for about 10 days and I was suffering from depression at the time (still am but whatever lol). I also go through periods of not replying to friends texts/calls when I feel particularly low.
I can't really explain why I did that as it wasn't really logical. All I can say is depression can be very difficult to deal with and its very possible that a guy in that situation isn't doing it in order to upset his girlfriend. Losing interest and motivation for things you usually find enjoyable is a common symptom of depression, I don't see any reason why social contact isn't included in that.
I always feel like I regret it after going through a phase of ignoring people. I didn't attempt to get back with the girl in question, despite the opportunity to do so. I kind of regret that now, although that is more to do with the drastic downturn in my success with women, rather than any strong feelings for that girl.