Its just because the first parts of the electrophoresis proces are not "clean" enough, still having more contamination etc that they dont respond well to the sequencing.
However: how come the fluorescence is more in the first bases ? Isnt the fluorescence staining done randomly or do you only stain the first bases?
And why are the first bases more packed?
as phage434 points out, early fragments are short and the gel in the capillary is optimized for larger fragments so separation is not as clear as later in the run. however, as i pointed out in an earlier post, you can obtain reliable data as close as 5 bases from the primer with better cleaning of the reaction products (at least, you can with the ceq sequencers, their gel formulations are different from the abi sequencers).
as for your questions regarding fluorescence...
sanger sequencing depends on dideoxy termination of the product nucleic acid. for the capillary sequencers, the dideoxy nucleotides are fluorescently labelled. once a dideoxy nucleotide is incorporated, chain extension is terminated. shorter chains are favored by this method although reliable data can be obtained for as long as 800 bases under standard conditions (more using modified electrophoresis conditions).
this wiki page can give you a better understanding: [url="http://en.wikipedia.org/wiki/Sanger_sequencing"]dna sequencing[/url].