That's one of the challenges of stripes!
It could be they are not perfectly straight in the first place.
When buying stripes, I look for a good quality fabric ... heavier, rather than lighter as I feel it will be more likely to hold it's positions, and less likely to get the wobbles.
Rather than "iron", "press" the fabrics ... when ironing, it is easy to stretch the fabric and then create the wobbles in the stripes.