It's really not a great surprise for a volcano in Japan to erupt; it happens all the time. This one, however, appears to have given little warning. A number of hikers were on the mountain when an ash cloud descended on them:
That cloud coming is really something you don't want to see up close and personal. If it's not hot inside (nuée ardente ), it's really sharp sand filling the air and choking you. But what's even more distressing is this tweet from someone at the summit 1 minute before the eruption:
The Japanese translates roughly as: "Enjoying Mt. Otake alone."