The explanation of what went wrong from the article:
Richard Brenner of the Clark County Fire Department said sodium hypochlorite went into the hotel's lazy river pool when a valve used to the control the chemical malfunctioned and remained on.
From a quick Google (I'm not going to claim to be a pool expert) it sounds like this is 'bleach' and is a standard chemical used in pools to keep the water clean. Oh, and it's generally not healthy for you in large amounts.