It is due to dominant/non dominant side muscle imbalance/flexibility. Equaling out your flexibility between right side and left side will help, as well as making your non dominant side more equal in strength to your dominant. One legged squats on a bench/box will help(get a lower box/bench when it gets too easy.) Add weight if it is too light (heavy chains draped over your shoulders works great, if no access to those a weighted vest or just something that doesn't throw off your center of gravity.) Lunges can work good as well.
Other exercises out there can help, really anything that isolates(sort of) each side of the lower body. Always start each set with your non dominant side first and equal the reps with your dominant side. Don't do more reps with your dominant side even if you can, you are looking for balance between the two.
With equal flexibility/strength on right and left side this problem will disappear.