Based on Mari-Lou A's helpful comment about checking the ngram references, I think it safe to conclude that the peaks (for me, the main reason to think it wasn't a misspelling on vocabulary.com) are caused by the following (unlikely) reasons:

An author with et viola in the name: (Alberti et Viola).

 

Books in the English corpus which are (partially) written in Latin. Most of the other results, mostly compilations of prose.

Based on Mari-Lou A's helpful comment about checking the ngram references, I think it safe to conclude that the peaks (for me, the main reason to think it wasn't a misspelling on vocabulary.com) are caused by the following (unlikely) reasons:

An author with et viola in the name: (Alberti et Viola).

Books in the English corpus which are (partially) written in Latin. Most of the other results, mostly compilations of prose.