When Manganese dioxide is heated with aluminium powder, the products obtained are-

On heating manganese dioxide and aluminium, aluminium displaces manganese from manganese dioxide to form aluminium oxide.

$3{\mathrm{MnO}}_2+4\mathrm{Al}\to 3\mathrm{Mn}+2{\mathrm{Al}}_2{\mathrm{O}}_3+ \mathrm{heat}.$

We will understand in terms of oxidation-reduction reactions. Aluminum is a strong reducing agent. It should be noted that aluminium precedes manganese in its ease of oxidation or reduces aluminium more than manganese. 

Therefore, aluminium is more reactive than manganese, which also explains why the first ionization energy of aluminium is lower than that of manganese. The lower the ionization energy, the more reactive the element.