Most larger pools these days use a salt-based chlorinator. However, I suppose this one could still be using the older powdered chemicals I used when I was a lifeguard. It’s large-scale chemistry, and all it takes is an error in the schedule or a malfunction in the automatic system to end up with a huge shock of chlorine in the water.
Most commercial public pools today either use sodium hypochlorite or calcium hypochlorite feeder systems, with some of the more sophisticated systems using sensors to dispense the appropriate amount of chlorine and acid needed to maintain proper balancing levels.
Occasionally a public pool will use salt for chlorination, but in most cases, the number of generator cells and the corresponding electronics needed for high gallonage pool becomes cost prohibitive.