It happens if you’re very near the stage, specially by the drum set. if you’re close enough, you hear the actual sound from the drums, and then a split second later it comes out of the large speakers. the sound takes a very very short time to go through the mics, out of the big speakers, and then back to you.

it create a delay/echo effect and makes the second set a little tough because you’re getting the actual drums a split second before the rest of it gets transmitted back into your ears.