Could have also had Maillard reaction during the boil. Extract brews are more prone to this than all grain brews. But all grain brews can be affected as well.
It's whay a lot of extract brewers (myself included) add half of the extract at the beginning, and the other half of the extract about halfway through the boil. Some don't boil ALL of their extract until about 20 minutes remaining in the boil.
Here is a quick explaination of Maillard's Reaction from a copy and paste from a BYO Magazine article:
Maillard Reactions
This is the name for the chemical reactions that result in the colors and flavors associated with crystal malts. The entire class of reaction, named after the French scientist who did much to unravel them, occurs in a wide range of food products, from baked bread to cured meat. Essentially, it is a chemical reaction between an amino acid and a reducing sugar. An amino acid is a naturally occurring acid that also contains a reactive basic amine group (this means it contains nitrogen).
Gary