That bastion of "quality" journalism, the Daily Mail, has an article about the Highway 3 landslide that includes a set of high quality images of the site:
The article correctly identifies the failure as being a dip-slope slide:
"The hill had a dip slop [sic - should be slope] on the side nearest the motorway. The other side of dip slopes are steep and irregular, while the slope itself makes it easier for rocks to slide down, experts said. The Ministry of Transportation said it as [sic] investigating up to 20 similar dip slopes near major roads in Taiwan."
A dipslope means that the beds in the slope were inclined sub-parallel to the slope surface, creating a plane of weakness on which sliding can occur, like this (from here):
The lack of a trigger implies a progressive failure - but I am surprised in that case that signs of distress were not observed prior to failure.
Finally, it is interesting that the Ministry of Transportation says it is investigating "up to 20 similar dip slopes near major roads in Taiwan". Having spent a great deal of time in Taiwan I would be surprised if there were just 20 such slopes, but maybe these are just the very large ones by major highways. The 18th August 1997 Lincoln Mansions landslide in Taipei, which killed 28 people, was also a dipslope failure, and the 2009 Shiaolin disaster was a wedge failure with a dipslope component.
The government really needs to get a grip of the management of slopes in Taiwan.