Though in the context of git, master is not intended to be a reference to master/slave [1], it is ostensibly widely mis-identified as such. The change therefore is being made by many as an attempt to be more inclusive. I pulled this [2] Bitbucket blog entry from the original article if you want to read what they have to say.
"ignorance is causing people to take offense when none was intended and instead of trying to educate people we can instead just make a greater effort to not offend them"