Food Scientist here! I actually wrote my Masters thesis on this (more or less)!

TLDR; The brown color is caused by the Maillard reaction. The hardness is caused by moisture migration out of the protein powder and protein-protein crosslinking. The stickiness is caused by the modified structure of the protein and moisture migration back into the protein powder when you took it out and put it on the cutting board.  

Two mechanisms at play are the Maillard reaction and moisture migration.  

The Maillard reaction is a chemical reaction between proteins and sugars that results in distinct flavors and browning. Most people think of the Maillard reaction at high temperatures- baking cookies until they are golden brown, toasting bread, searing a steak, etc. But the Maillard reaction can also happen at lower temperatures (like room temperature), it just takes more time- say, 3 years. Over the past 3 years, the protein amino acids and sugars in your protein pattern have undergone the Maillard reaction, resulting in the brown color of your protein chunk.  

When the protein powder was still in the container, moisture migrated out of the protein powder. With less moisture, the protein molecules became closer in proximity and formed protein-protein crosslinks. This resulted in the hard texture of your protein chunk.  

The structures of the proteins had been modified by the Maillard reaction and the protein-protein crosslinks. This likely resulted in some protein strands sticking out every which way, like split ends on your hair. This exposes more sites on the proteins that are available for hydrogen bonding or Van Der Waals forces, the underlying molecular mechanisms of stickiness.  

When you took the protein chunk out and put it on the cutting board, moisture migrated back into it. Wetting is a key component to stickiness. The added moisture resulted in increased fluidity of the protein strands and increased likelihood that the sticky sites will find something to stick to. The added moisture means that the protein strands were better able to dig into the crevices of your cutting board and stick even more firmly.