Perhaps, did wash down the sides of your pan as you started to heat the sugar--Perhaps you missed some sugar grains, or perhaps you did not wash down? And if you stir it too much, melted sugar can get up on the sided of the pan, re-crystallize and then can "contaminate" your caramel with "seed" crystals, which will encourage your caramel to crystallize.
There was also info on the following pages about warming the cream before adding to the melted sugar so as to prevent crystallization.
Back in 2008 when I was having very inconsistent results making the caramel layer 4 charlie's pie &
was on the verge of buying the latest edition of CARAMEL-MAKING FOR DUMMIES, I believe it was he who suggested I add a smidge of lemon juice to my caramel to help avoid crystallization (or maybe 'twas someone else at eat.at and if so, I apologize for not giving credit where credit is due), but ever since that tip, I've never had a problem with turning out perfect homemade caramel. See the following finecooking video and recipe for further explanation==>
And in case anyone is interested, here is Charlie's REC for Caramel-Pecan Black Bottom Pie--oh, yum!--http://www.eat.at/swap/forum/index.php?action=display&forumid=1&msgid=91210
I did wash down the sides, but wouldn't the vanilla bean have added *impurities* to my sugar?
It even looked different...turned a yellowish tinge almost immediately and started to crystallize almost immediately. I was shocked it did that so quickly. And brushing them down with water didn't make any difference. They just grew back in to a floating sugar floe. Dumped the whole thing and started again with plain old sugar. Worked fine...had caramel 30 minutes later.