Q:

What causes wort darkening in the kettle? 

What reaction is happening in the wort to cause darkening?   Maltose for example doesn't caramelize until 365F.  Maillard reactions are another source of non-enzymatic browning, but they require low moisture, alkaline conditions and occur at 310F (on average). 
So why does my wort get darker as it boils? 

A:

Despite the Wikipedia page for Maillard Reactions currently saying that Maillard Reactions require low moisture, alkaline conditions, and temperatures well above the boiling point of water, Maillard reactions can happen outside of those conditions.   For instance, tanning lotions utilize a Maillard reaction, and I'm fairly certain women wouldn't use them if they had to heat their skin over 300°F. 
I believe the phrasing about low moisture, alkalinity, and heat applies only to carmelization, a specific type of Maillard reaction.   Furthermore, the source that the Wikipedia article cites in that paragraph, http://albumen.conservation-us.org/library/c20/reilly1982a.html, is about Maillard reactions in egg-white photo prints, and has nothing to do with high-temperature reactions. 
In short, wort darkens due to a combination of Maillard Reactions and help from concentration of the wort due to boiling off some of the water volume. 
Bonus points if someone fixes the Wikipedia page.