I found the same recently in a fell race.
Uphill puts a lot more strain on your back because you are extending your thighs much more... a bit like doing agressive squats with weights on your back in the gym.
I agree with LozF. Stretching, using the gym ball (reverse reaches/stretches), and regular lower back manipulation or exercises will cure the problem.
It really worked for me. I've had a bad lower back for 20 years because of rugby. In the past 3 years I cured the problem myself just by doing exercises at home.