Back Pain

Back pain when cycling can be caused by a number of things.
The most common are.

Excessive reach to the handlebars
Too large of a drop from saddle/bars.
Saddle too high
Leg length discrepancy
Excessive reach to the handlebars.

Excessive reach to the handlebars stretches the body outside of its natural comfort zone and puts a huge amount of stress on the lower back and shoulders.

Drop from saddle/bars too great.

This puts a lot of pressure on the lower back and shoulders and causes the back to arch creating bad riding posture. It can also put to much weight on the hands creating numbness after a while in the saddle. It is important to remember that just because you are low at the front it doesn't necessarily mean that you are aerodynamic. If your body is not naturally flexible or you suffer from bad posture especially in your lower back then you could be creating a wind trap or parachute effect on your body. Remember the rule adjust the bike to the body and not the body to the bike. Everybody is an individual and no body is exactly the same, if you have two people who are 5.10" in height that doesn't necessarily mean that they will ride with the same position or even ride the same sized bike. There are many factors to consider.

Longer total leg length = Higher Saddle height
Longer Femur length = More Saddle set back
More back flexibility = bigger drop to the bars
Shorter torso = Shorter reach (shorter stem length)

Back issues from injury etc can cause a huge difference in position with the saddle to bar drop being almost 0 in some cases with the aim putting as little pressure on the back as possible and making a long ride an enjoyable experience and not a painful one.

Saddle too high.

This causes the hips to have a side to side movement and puts excess stress on the lower back and around the pelvis.

Leg length discrepancy

If you have a leg length discrepancy (one leg shorter than the other) then this can cause certain problems with the lower back if one of your legs stretches at the bottom of the pedal stroke due to it being shorter then this can have a similar effect to having your saddle too high.