There is the toxic algae concern. These deaths may be related to that, either directly due to ingesting the algae, or indirectly due to avoiding the water. But the investigation is ongoing, so "unknown hazard" it is.
@sfinkz has said that they expect an influx of the curious when it is opened, and it's often hot well into October.
NOAA predicts a high of 87F/30.5C for Mariposa on Friday, interestingly, 93F/33.9C is predicted for the SLT switchbacks. I share the opinion that many have expressed, that it's an area to avoid when it's hot. I'm sure the prospect of curious people exploring the area (where are them gold mines?) while the weather is still hot gives the authorities pause. I'd go with "closed due to an unknown hazard" too, if I were them. MOO.