I would guess that the height of the step is causing you to step up with bad form, arching the back, perhaps?
Try building up to 12 or 14" over time. Start with a height that causes no problems (even if that's 8"). Be sure to keep your core strong (feel like there is a corset snuggly around your abs and lower back as you move). Move up in height 2" at a time. Back down if your back causes problems again.
It might also be a problem with your general core strength, especially your lower back. Doing some planks can help(on elbows, body straight, do some during commericals and try to hold each plank as long as you can with good form, doing the "corset tightening" move when you feel you can't hold any longer to get a few more seconds in. Drop down to knees to rest shortly , then go back into plank, repeating as often as necessary until the end of the commercial break. As you progress, try to hold each individual plank longer.)
"Birddogs" are also a good move for increasing strength and endurance in the spinal erectors. On all fours, lift right arm and left leg parallel to floor and reach outward with fingers and toes. Again, hold as long as you can, and build up to more. Be sure to work both sides!