1931: The year that "to-morrow" lost its hyphen [Google Ngram Viewer]

Like tomorrow, today and tonight went through the same steps, both beating to-day and to-night around 1930--1931. 1 2 Does anyone know what happened in those forty years of 1900--1940 to make this so? For ever lost out to forever in 1886. 3

Roots:
1. Books.google.com,. 'Google Ngram Viewer'. N.p., 2015. Web. 30 Apr. 2015.
2. Books.google.com,. 'Google Ngram Viewer'. N.p., 2015. Web. 30 Apr. 2015.
3. Books.google.com,. 'Google Ngram Viewer'. N.p., 2015. Web. 30 Apr. 2015.

/r/etymology Thread Link - books.google.com