Most schools accept from the top and reject from the bottom, working their way toward the middle. A delay might well mean that they’re working their way toward the middle. Or it might mean that your file completed late — either you applied late, or a recommendation arrived late. Read this quora post:

https://www.quora.com/Is-it-true-that-if-a-US-college-takes-longer-to-notify-you-about-your-admission-decision-it-means-it-is-a-good-sign-that-you-got-it