It looks like your nut doing most of the load is not torqued down? You need to preload the connection to counter the lever arm of the weight on the light, not just finger tight.
From the pictures it looks like it has to do with the junction between the vertical and horizontal bar. Like theatrus pointed out, it looks like the nuts in the rear could be tightned more which would cause the horizontal arm to pivot upward. If there is some mis-alignment from the L shaped pieces that you have bolted to the vertical bar you could try 'shimming' it out with some washers to compensate. For example when you fully tighten all those bolts if its still leaning down you could add washer(s) between the horizontal bar and the L shaped pieces on the front.