We’ve heard on very rare occasions about bees chewing at the timber hive.
Any thoughts on why they might do this?
We’ve come up with a bunch of theories but I don’t think it’s happened to any of our hives here so we were curious as to whether anyone has any first hand experience of this and more educated observations on what the cause might be.
It is a part of a bee colony natural behaviour. When colony moves into a tree hollow, they remove soft rotten wood starting from top of the hollow. Propolising it is the next step.
Something may trigger this instinct in hive as well. Wood rot, for example…
Sometimes I’ve seen entrance reducers chewed in a way to open it more and round off the edge. An obvious message in this case: “the flow is ON, lady - let us at it!!” 
Great question. Hope there is a fascinating reason. Never gave it much thought.
I chew pencils when I’m trying to work out Excel 
Haha yes maybe it means they’re trying to focus 
Thanks all, some other thoughts we had were that the bees may be tidying up an area chewed by mice or other visitors, trying to access something inside the wood or a particular product used to treat the timber, or working to improve the ventilation of the hive in high temperatures.
Hopefully not wood rot as @ABB mentioned but happy to go with @Eva 's scenario 
Like @Eva, I have seen bees chew away at entrance reducers made of pine or oak. However, I have never had bees which chewed cedar. Yet… 