Why does human chromosome 19 have the second highest number of 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.