Whiplash during ride on roller coaster?

Since I was little, I have been going to Six Flags Great America just about every year. While most of the big roller coasters, such as the Viper, Superman, and Raging Bull, have a smooth ride, I have noticed lately that the Demon and Iron Wolf roller coasters have a rather bumpy and whiplash causing ride, and in fact yesterday when I was at Great America, I nearly got whiplashed pretty badly while riding the Iron Wolf roller coaster. The Demon, Iron Wolf, Raging Bull, and Superman are all the steel frame type coasters with the steel bar type tracks. Out of curiosity, what causes some of those roller coasters to have that whiplash type of bumping around during the ride?