My guess would be incomplete first developing or incomplete bleaching. The first developer has to convert all the exposed silver halide in the very-well-exposed leader to metallic silver. The bleach has to remove all of the 'black' silver, leaving just the undeveloped white silver halide in the emulsion.
I've done my reversal work using sheet film and I turn on the room lights when the film enters the bleach. This way I can make sure the bleaching has finished and it also insures complete 2nd exposure. Bleaching can take much longer on heavily exposed film - like the leader - than it can on regular negatives (er, positives). Film can appear clear in the bleach even though there is quite a bit of residual silver halide in the emulsion - if you look at the negatives after the bleach step it looks like they are quite devoid of all highlight detail, but the detail re-appears in the 2nd developer.
Another mechanism may be that the heavily exposed leader is partially solarized - exposed so much that it is reversing a little bit all on it's own - something that would never be noticed in regular processing. Try giving a frame a Zone XII exposure - enough to be completely clear, but well below any self-solarizing threshold. If this frame comes out clear, without fog, then I would not pay an more attention to leader density.