Q:

"Hence" and "hence why"

My question is, is the use of the word "hence", used in it's most common sense as an alternative to "therefore", strictly acceptable in English usage in the following example:

I like bananas, hence why I eat them.

I see a lot of people using the word "why" after the word "hence", and I have always considered it, at the very least, inefficient use of English. Happy to find out other views. Many thanks for all comments.

A:

It appears that this construct used to be a lot more popular in the 1800s and continues to be used today. Going by Ngrams:

Excerpts:
Barbara Heyman, 2012:

I looked at it later in 1922 with the idea of arranging it for violin, cello, and piano. Hence why we still call it the “Trio”.

Thomas A. Blackson, 2011:

The details of his pursuit were so undefined it was difficult to understand how the pursuit was supposed to work and hence why anyone should value it so highly. 

Marcel Fafchamps, 2004

This may explain why the threat of stigmatization is largely ineffective against so-called informal sector firms which, as a rule, are not registered — and hence why transactions among informal sector firms remain quite unsophisticated.

In other words, this usage appears to be quite valid.

