Why can 'kick back' mean 'get relaxed'?

"Kick back" can literally be used to describe the action of putting your legs up when you are sitting in a chair. So, if I "kick back" at my desk at work, I put my feet onto my desk, like this:

This literal idea has been extended to talk about relaxing, sometimes even when no actual "kicking back" takes place.

As this NGram shows, "kick back and relax" has shot to prominence in the last decade or two.

But searching Google Books for "kick" shoes "and relax" from 1950-1990, I get about as many hits as I do for "kick" back "and relax".

I'm not going to analyse all the hundreds of instances of either, but I can easily see that both those two search terms often match the same citation. What this tells me is that the "original" version would have been something like...

Kick off your shoes, lie back, and relax.

...that's to say, it's something you do when you come home from a hard day's work (esp. if your job involves long periods standing on aching feet). But I do accept that in the minds of many today, it applies equally to "kicking back" in an office chair and putting your feet up on the desk (normally without taking your shoes off! :)

Idioms don't have to make sense. They become their own definition and analyzing the individual words literally will never make sense. They become acceptable to us through repeated usage, to the point where our brain just accepts them without thinking about it.