The "mushroom" shape, formed at the end of a candle wick after burning, is the result of carbon buildup, where the flame consumes more wax than it can burn. It is a common occurrence and can be a sign that the wick is too large for the candle. Keep in mind that some wick series, such as the CD wicks, are prone to mushrooming more frequently than other wicks.
If "mushrooming" occurs with your candle, we recommend trimming the carbon buildup off the wick between burnings. If you are seeing a combination of excessive mushrooming along with soot building up on the sides of your container, we would recommend wicking down one wick size to help control the burn.