If your candy crystalized -- turned back to sugar -- then either you stirred it during the cooking or sugar crystals got into the mixture.
Looking at the recipe (at Epicurious) two things jumped out at me.
1) You should never add a hot mixture (the caramel) to a cool or cold mixture (the dissolved sugar), so make sure that sugar mixture is good and hot before you add the caramel.
2) When making the caramel the sides of the pan should be brushed down at the beginning, not the end of cooking (you can avoid this by covering the pot for one or two minutes, letting the steam wash it down, too).
Also, very important in making candy is not disturbing the pan once the sugar starts to cook. From the moment your combined mixture goes over the flame until it reaches 160 degrees you shouldn't touch it at all, doing so could release the suspension and the result will be grainy or crystalized candy. Myself, I don't like to cool my sugar mixtures in the pan. I pour it out on cool marble, wait for it to cool completely, then kneed with metal spatulas -- difficult to do at first but well worth learning.
Lastly, never scrape the pan. You should expect that some of your mixture is not destined to become candy. Adding those last bits dramatically increase the chances of crystalizing your candy, or just as bad, adding burned bits.
Hope this helps.