grammaticality - "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.