I have found that by the time all the alcohol has evaporated, my fond turns from beautiful brown to charcoal black.
Why does it happen?
Prior to adding wine (or any liquid), you have browned meat and/or vegetables. Ideally, you are looking for a dark browning. Once you've removed those items from the pan, you add wine or other liquid. It should begin to boil. Immediately begin scraping up the fond (the brown bits that have stuck to the bottom of the pan). They should scrape up with a little force. It is best to use a straight edged wooden spoon. Once the fond is scraped up and incorporated into the liquid, there should be little chance of burning, unless you evaporate all of the liquid. You can also turn the heat down a bit for more control. Why does it happen? Maybe your heat is too high. Maybe you are not scraping all of the fond off the pan. Maybe you are not using enough liquid. I would bet there is some fond left on the pan, which has not been fully incorporated.