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.
1. https://twitter.com/xpasky/status/1271477451756056577?s=20
2. https://bitbucket.org/blog/moving-away-from-master-as-the-de...