Over 30 Hikers Die During Ontake Eruption in Japan: What Happened?

Over 30 Hikers Die During Ontake Eruption in Japan: What Happened? | WIRED: 
The question now is: what happened? 
Japan has one of the most sophisticated volcano monitoring networks in the world but from what it seems, this eruption was completely unexpected. 
Did someone miss a sign or did the network fail? 
My guess is the answer to both of those questions is “no”.

Based on what I’ve read and seen (and this is speculation on my part), this eruption may have been a steam-driven explosion known as a “phreatic” eruption. 
TheJapanese Meteorological Agency suggested that even compared to a smaller eruption in 2007, this explosion (see below) had almost no warning. 
This occurs when water seeps into the cracks in the crater area of a volcano and gets hot enough to flash to steam. 
This rapid boiling causes fracturing of the rock and explosively ejects material out of the crater as the pressure inside the crater or conduit goes up exponentially. 
This explosion then produces ash clouds and pyroclastic flows made mostly of the shattered debris of old rock at the crater rather than new magma. 
Although phreatic explosions can be small, they don’t necessarily need to be as they can even become “phreatoplinian“, with ash columns that reach tens of kilometers into the air above the volcano.