Why would magma have high amounts of nickel and chromium?
The current thinking according to modelling and previous observations is that the high amounts of $\ce{MgO}$, $\ce{Cr}$ and $\ce{Ni}$ is due to the magma feeding the Mt. Lamington volcano being contaminated by the nearby Papuan Ultramafic Belt (Smith, 2014; Arculus et al. 1983).  The Papuan Ultramafic Belt is, according to Lus et al. (2001), a early Neogene (Tertiary) ophiolite.  The region itself is not associated with an active Wadati-Benioff Zone (Smith, 2004).

Smith describes that the composition of the magma that fed the Mt. Lamington eruption represents part of an unusual tectonic environment, as described in part in the answer to the earlier question What are the geological mechanisms for sea floor spreading in the Bismarck Sea?.  Smith describes that the magmatic geochemistry is influenced by the variables of crustal thickness, tectonic setting and the geochemistry of the lithosphere.

Finally, the relationship between the tectonics of the region and magmatism of the Papuan arc environment has been described as a 'delayed melting' as a partial melting of a source that was


  metasomatised in an earlier convergent event.


The complicated geology is shown in the image below:



Tectonic and geological map from Syracuse University Department of Earth Sciences, the ophiolite (PUB) outcrops are in purple.

References

Arculus et al. 1983, 'Ophiolote contaminated andesites, trachybasalts and cognate inclusions of Mt. Lamington, Papua New Guinea: anhydrite-amphibole bearing lavas and the 1951 cumulodome', Bulletin of Volcanology and Geothermal Research 

Lus, W. et al. 2001, 'Papuan Ultramafic Belt (PUB) Ophiolite: Field Mapping, Petrology, Mineral chemistry, Geochemistry, Geochronology, And Experimental Studies Of The Metamorphic Sole', American Geophysical Union, Fall Meeting 2001, abstract #T52D-09

Smith, I. 'High magnesium andesites: the example of the Papuan Volcanic Arc' in Gomez-Tuena et al. (eds) 2014, 'Orogenic Andesites and Crustal Growth', Geological Society Special Publication 385 page 128