I have a different theory, although the TV show theory is certainly a plausible one. My theory is thus:

George RR Martin contracted an illness at some point in the early 2000s. This illness exhibits none of the symptoms common to most illnesses; there is no vomiting, nausea, headache, muscle soreness, or skin irritation. As far as medical professionals have been able to determine, there is no observable impact on the immune system at all. In fact, there is only one observable symptom: upon publishing a book after contracting said illness, it becomes exponentially more difficult to publish each subsequent book. Since this is a symptom can only be observed in a small percentage of the population (published authors), it is unclear what percentage of the total population has this illness. Further, it is unclear how this was initially contracted, whether it is viral or bacterial in nature, or whether the human body can develop antibodies or in any other way fight off this illness. This illness is communicable through direct contact, although further study has to be done to determine if airborne transmission is possible. What is known is that George RR Martin became a host for the illness and transferred it to Patrick Rothfuss during a chance meeting at a convention in 2010. Patrick Rothfuss then transferred it to Jim Butcher in 2013, this is suspected to have also occurred at a convention, although that hasn't been confirmed. At this point in time, so little is known about this disease that the only real solution is to prevent contracting it. Due to the common demoninator of these cases, it is recommended to avoid conventions or other large gatherings of people who may have been in contact with these three known hosts. If this can't be avoided, wash your hands regularly with a solution of atleast 30% isopropyl alcohol to reduce your risk.