Sometimes the obvious, self-evident explanation is the correct one.

I've looked at these pictures a bunch of times. Damn.

Look in pictures 5-7, at how many of those slides started at roads. At a road ecology seminar, the speaker explained it to us. The road constructors cut away the back slope, leaving it steeper than the ground line, hollowing out the hill to collapse. They put any excess on the fill slope, which is too heavy and slides downhill from that.

(picture from an FAO report on roadbuilding.)

Then, in picture 8, that bridge probably wasn't undersized for the original creek flow. The pre-logged hydrology was forested; water infiltrated the soil before running off. Water would still move downhill through the soil to the creek, but slower. The flows might run big for longer, but the biggest flow was never like you see in picture 8.

Everything is connected. If you mess with a physical system, the costs will ALWAYS come due.