Why does straw steam whilst shavings don't?

The muck is biodegradable and is rotting away. One of the by products is heat as in a compost heap.

I agree that it is the heat of composting. When you have bacteria and other microorganisms breaking down organic material, heat is released. The more biological activity, the more oxygen supplied, and the larger the pile of compost, the more heat. Some compost piles can get so hot that they spontaneously combust!

So that still leaves the question about why your straw steams but not your wood chips. I'll give you a few ideas, but leave the rest up to you. Composters typically talk about an ideal ratio of carbon to nitrogen (CN) ratio to maximize biological activity. Straw and grass and anything that used to be green has a lot more nitrogen than wood. If you add nitrogen fertilizer to your wood chip pile, it might get hot, too. Wood also has lignins and tannins which can be difficult for bacteria to break down. It could also have to do with aeration. Perhaps your straw pile is more "fluffy"? I'd bet it's the CN ratio that makes the biggest difference, though. Add some N and find out.