I think it's a single ply or loosely plied so may untwist easier. Watch how you turn at the end of a row that you don't go the same direction all the time, and if you do LT CO you might have to let go of the thumb yarn to let it wind up again. Also the way you hold and wrap the yarn might untwist it somewhat.
I would also suggest you use a larger needle too, because you'd probably get the same gauge single stranded on the 10½s you are now with it doubled. It's really a light bulky/aran not a worsted, so doubled up would be more of a super super bulky and the gauge would be something like 2-3 sts per inch which you should get on size 11 or 13. A true worsted doubled would be a super bulky in the first place, not a bulky.
sue- knitting heretic