It’s a reference to classic Mega Man games!
Mega Man’s death comes with an explosion of orbs in his home games, just like in Smash Bros. Nice little reference to canon, eh? We can only hope other characters will get this same treatment. Perhaps knocking out Robin will be accompanied by this music and Chrom shouting “No! Not you, Robin! Open your eyes! OPEN YOUR EYES!”
We’re being sarcastic that would be awful in Smash Bros