Many ab exercises put stress on the lower back, so what you're experiencing isn't abnormal. If you're doing abs from the floor, you reduce lower back stress by bending the knees, and further reduce it by resting your calves on a bench and just doing "crunches." Any kind of straight-leg raise from the floor puts stress on the lower back as well. I think hanging leg/knee raises don't stress the lower back too much.
Frankly, you should be strong enough to do any of those exercises though w/out too much back trouble. So, you probably need some lower back strength. Also, you don't have to do 30 reps of everything, which can lead to fatigue kicking in and your back coming into the picture to support your tired abs. You can do sets of 8 or something, and use weight if you're currently doing sets of 25 or something dumb like that. LoL