Answer: A large housing development was built in the area, making the slope unstable.
Step-by-step explanation:
The Gros Ventre Slide landslide of June 23, 1925, was one of the largest fast-moving landslides in generations and it occurred near the village of Kelly, Wyoming.
In just three minutes, huge amounts of rock and debris cascaded down the north slope of Sheep Mountain.
The first 3 options listed in the question were listed as official factors contributing to the Landslide.
Heavy rains may have indeed made the materials heavier.
River erosion as well may have indeed made the slope unstable.
And Rock layers were dipping toward the valley, parallel to the slip surface.
There is NO evidence that a large housing development being built in the area made the slope unstable.