I think it is a cyclical process. Having more subscribers means that you have a larger base audience when you release a video. Since that base audience is likely to see your video posted, you have a good chance at getting a reasonable number of views in the first 48 hours. Getting a lot of views in the first 48 hours will improve your ranking for that video in YouTube's algorithm, making it more relevant in searches and related content, hence bringing in more views. The more views your video gets, the more likely you are to gain more subscribers from it. Then you're back to step one with an even larger base to start from since the last one.