I can get brief lower back pain right after "mashing" up a steep climb in very low cadence and high pedal force.
My usual climbing is a faster spin on moderate grades (and 34 front - 32 rear low gearing). Then stand up and slow as much as practical to keep the force on the pedals reasonable.
And riding a lot: 35-50 mile rides at least a couple times a week. 3500 to 5000 miles per year.