According to this review answering such questions in the movie itself would have "ruined" it. The original script contained an explanation of the start of the time loop - a voodoo spell - but that was considered a poor idea.

So, I wonder if there is a conclusive answer to your question. It was deliberately left out by Harold Ramis. We are left speculating about it (maybe even encouraged to do so) and I think that is one of the reasons why this movie has had so much impact.

According to this review answering such questions in the movie itself would have "ruined" it. The original script contained an explanation of the start of the time loop - a voodoo spell - but that was considered a poor idea.

So, I wonder if there is a conclusive answer to your question. It was deliberately left out by Harold Ramis. We are left speculating about it (maybe even encouraged to do so) and I think that is one of the reasons why this movie has had so much impact.