Brazil's not so much an antagonist, but the ruling party under Lula da Silva is traditionally associated with workers parties in Cuba and Venezuela. Brazil has also moved toward increased cooperation with China and Russia in defense and industrial manufacturing, as well as considered aligning its considerable influence on the oil market with OPEC, and is under constant lobbying from Venezuela to do so.
Most importantly, the United States would very much prefer that no power centers of comparable strength arise in the Western Hemisphere, and Brazil's GDP is closing in on France.
Powers in relative decline are always suspicious of rising powers, especially if they could potentially one day exert significant influence in a nearby region.
Foreign policy's a zero-sum game in the IC and defense world.