He's allergic to mangoes—red rashes start erupting on his skin as soon as he eats them.

erupting 是动词 erupt 的现在分词形式。erupt 由前缀 e-（表示"向外"）和拉丁词根 rupt（表示"破裂"）组成，本义是"突然爆发"，在这里指"（皮疹）突然出现"。erupting 描述的是过敏反应导致红疹突然在皮肤上冒出来的状态。