I actually do this same thing unintentionally to people. It's not nice, I know, but I can't seem to help it. They are people I care about, too. I'm just one of those people that you have to contact if you want me to talk to you. My brother and sister are the same way. I hadn't spoken to my brother in literally months or maybe even a year, and then we started talking again on a regular basis a few weeks ago.
On my part, I can't speak for my siblings, a lot of it has to do with a poor concept of time. I think only a few weeks have gone by and it has been a month or two. I cannot seem to change this no matter how hard I try. Another thing is that I moved to a different town, and there is really a case of out of sight, out of mind going on.
But mostly it all just ties back in to the "I don't reach out and contact people and I don't know why" factor.
I tell you all of this to say that it may not be intentional on your friend's part, and I doubt very seriously that it has anything to do with your writing. Have you tried contacting them again? If this person is someone who is important to you then you might have to do the reaching out. I know that life is short and sometimes it is not worth the effort to you, but there are people out there like me who, while we care very much for the people in our lives, don't reach out to those people for whatever reason.
Of course the other two options you have are that they really are jerks, or that their PhD program is consuming their life at an alarming rate.
Either way, I would say confront them about it or stop worrying. If you confront them and they are like me or just really busy then they will gladly explain to you that it wasn't intentional, etc. If you don't value their friendship then don't worry about it.
At least then you would get some closure.