What makes the conditions at the core/mantle boundary ideal for aluminum to combine with other elements besides oxygen?
A reason found experimentally is that the pressure and temperature conditions at the mantle-core boundary result in the reduction of aluminium oxides.  According to the Nature article Chemical interaction of $\ce{Fe}$ and $\ce{Al2O3}$ as a source of heterogeneity at the Earth's core–mantle boundary (Dubrovinsky et al. 2001) through their experiments simulating the thermochemical conditions at the boundary, that one of the main factors in the core-mantle boundary region is 


  that iron is able to reduce aluminium out of oxides at core–mantle boundary conditions, which could provide an additional source of light elements in the Earth's core and produce significant heterogeneity at the core–mantle boundary.


Further, from the ESRF Highlights 2001 webpage Chemical Interaction of Iron and Corundum at High Pressure and Temperature: Implication for the Earth's Deep Interior, experimentally found that the following reaction, forming iron-aluminium compounds (amongst other alloys) occurred at temperatures that are believed to exist at the mantle-core boundary:

$$\ce{($2+3x$)Fe + $x$Al2O3 -&gt; 2FeAl_{$x$} + $3x$FeO}$$

Where $x$ varied between 0.02-0.03 to 0.25