While chromosome 19 only is the 19th largest autosomal chromosome, it contains 1440 protein-coding genes, and thus has the second highest number of protein-coding genes of any human chromosome.
For comparison: If one would naively assume the same density of protein-coding genes as on chr1, which has the highest absolute number of protein coding genes (2109), the anticipation for chr19 would be ~540 protein-coding genes.
This Nature paper from 2004, by Jane Grimwood et al. goes at least a long way towards giving an answer to the question of the OP. In short: there were inordinately many duplications, especially during an event 30-40 million years ago, as well as during a much more recent event. These duplications are, uncharacteristically, predominantly intra-chromosomal rather than inter-chromosomal. Also, chromosome 19 contains a lot of immunoglobin-like paralogues: a type of gene for which it is clearly evolutionarily adaptive to undergo rapid duplication followed by random mutation, as they play a role in adapting to potential antigens.
It's interesting that not only the leader 19, but also 16 and 17 follow a similar trend. Perhaps their size could be the best weight/length proportion to ensure a safe replication? Then what would have to be explained would be 18, so far to the left. That could be if 18 is newer, resulting from the split of a larger chromosome or the fusion of two smaller, having yet no time to accumulate a greater number of genes favoured by the advantages of the "genes positional co-evolution" (if such thing exists).