This is the million-dollar question, and I don't think anyone knows for sure. There are a few likely factors, though, that probably contributed to that rough final stretch of the season.
First, although Notre Dame was lucky not to have basically any major injuries this year, by the time they got to the last quarter of the season, a number of Irish players were banged up, including running backs Josh Adams and Dexter Williams, wideout Equanimeous St. Brown, tight end Alize Mack, cornerback Nick Watkins, etc. The schedule ND played this year was ridiculous, with games against teams ranked Nos.