stretch additive for sure, as these are essentially stretch garments via the rib (and the fact that the younger ladies love to buy them tight!)
But, how are they cracking? is it just with normal stretch test stretching, or rigorous tugging imitating the abuse inflicted on said garment when a size 12 is squeezing into a 6?
If they crack with only modest stretching, you know the answer.
You can also do the true cure test and launder a couple of times.