In many venues that were not designed with dedicated power for sound systems, it's common for all of the receptacles in a room to be on one (or maybe two) circuits. That's because they are installed with just one purpose in mind: a place to plug in the vacuum cleaner, and there needs to be just enough so the janitor can move about the room without using an extension cord.
Commercial building designers often forget to consider occupant use when planning the electrical. That means that the electrical isn't planned; rather, the electricians install the minimum required by code. Which is to support vacuum cleaners and building maintenance.
At least that's the way it is in the gymnasium of a school that I often work in. It's troublesome, because they like to have catered events there, which means that they want to connect coffee pots and portable lighting in addition to the sound system.