Through which mechanisms did the Drosophila Histone Cluster evolve?
This is the best review I have seen with a comprehensive discussion on gene duplication and amplification mechanisms (and it is also quite recent). It seems like there may be no definitive answers to your question in the literature--perhaps because of a paucity of viable experimental models that would allow one to test various hypotheses.

Also @canadianer is quite correct. Most metazoans with sequenced genomes have some kind of clustering of their histone genes (as well as multiple copies). In Drosophila embryogenesis the early zygotic cell-cycles are extraordinarily brief, on the order of 20 min as I recall, which means the fertilized egg is essentially a bag of DNA polymerase replication complexes.  Therefore there is a very high demand for histone proteins.  The gene amplification you describe is one way to achieve high concentrations of histones in a short time-frame.

Note that not all metazoans undergo this rapid burst of synchronous nuclear division, and therefore not all species have such a high copy number of core histone clusters.