I don't think Boston has or had anything to do with how fast Disney races are selling out. I think it's more likely due to several other factors...
1) running distance races is getting more popular across the country
2) people are tired of just a normal race and are looking for new & different experiences (Goofy, Dopey, color runs, mud runs, etc.)
3) destination races like Disney are easy ways for a runner to get away to a race with non-running family members
4) races with generally good reputations for treating their runners well, having good course support, and solid organization are always popular
5) Disney offers unique and interesting medals. Lots of people run for the bling.