Most of T-Series' subscribers come from people creating accounts in India and getting automatically subscribed to T-Series by YouTube's algorithms.

YouTube did this a while back with other demographics. For instance, CaptainSparklez often mentions this as one of the reasons why he became big; i.e. YouTube was autosubscribing folks to certain creators.

EDIT: If I recall correctly, when you create a new YouTube account, they give you a list of channel suggestions that you can check off to bootstrap you into suggestions based on your preferences, so this could be that, meaning people just "select all".