Q:

Why is こんばんみ used as a response to こんばんは？

When I chat the term こんばんみ is occasionally used as a response to こんばんは. Is there any particular reason why this word is chosen?

A:

「こんばんみ」 is a greeting presumably created and definitely made popular by comedian ビビる大木 a couple of decades ago. 
As always, some people like to mimic whatever schtick they hear on TV that they find "cool" or simply "new".
「こんばんみ」 was even more popular a decade or two ago than it is now.  I was a bit surprised to hear you still hear/see it often enough.
Here is the top page of Ooki's official blogs and you already know what he says at the very beginning!

