I found this online.
Carrots are a cool-season crop. They are sweetest when daytime temperatures are between 60 and 70 degrees and nights are in the 50s. As the temperatures rise, quality is reduced. These vegetables also prefer a loose sandy-loam soil. Soil that is high in clay may be part of your problem.
As the plant and top growth mature, the carrot also grows in size and tries to expand. In heavier soils, the roots push upward instead of downward. The exposed part receives sunlight and develops chlorophyll, causing the top of the carrot and core to turn green. The green parts have a bitter taste. Soil that holds moisture for prolonged periods will also cause problems, including poor colouration.
I've also found that lack of trace nutrients and clay soil affect their taste from my experience. I side dress with Soil Alive and make sure my soil is amended well. A frost will sweeten carrots sometimes.
