I remember myself going to an open-air concert where the same thing happened, and I know exactly why.
There were I think six or eight loudspeakers positioned at the edge of the audience area in a full circle, pointing inwards.
This might seem to concentrate the sound into the circle and thus be very efficient. Remember that sound is easily lost outdoors so you don't want to waste any power on the birds in the trees.
However this arrangement only works for people at the very center of the circle.
Anywhere else, you will hear the loudspeaker closest to you loudest. Then you will hear all of the other loudspeakers, less loud according to their distance, and delayed also according to their distance.
Sound travels around 30 cm (1 foot) per millisecond, so you only need a distance of 15 meters (50 feet) or so before the delay becomes noticeable. Any more than that and the echo becomes distinctly irritating.
The problem on this occasion was so bad that several people went up to the mix position to complain. In my experience, this is extremely unusual. However once the show had started there was nothing that the sound crew could have done.
The solution to this problem is that for a show 'in the round' the loudspeakers should be in the center, pointing out. In this way, no-one will hear sound from more than two loudspeakers (if they are exactly on the mid line between two cabinets) and since both of those loudspeakers are at the same distance, there will be no delay. Most people will hear sound from one loudspeaker only.
For more information, look up 'center cluster', which is a loudspeaker array that uses exactly this concept.
In a large stadium it may not be practical to have a cluster exactly at the center. In this case the loudspeakers should be in a circle entirely inside of the audience, pointing out. As much as possible each audience member should hear the output of only one speaker.
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