I wonder this too. Vancouver Island was hit by a drought this summer and the (not unusual) torrent caused "water to flow where it has never flowed before" according to the road engineers.
Soil biology does some weird thing to soils in the boundaries.
Fungally dominated soils have a greatly increased mineral and water availability due to the meddling of the fungi in soil chemistry. But if you dry out fungi enough, they become hydrophobic, like sphagnum does, and this can increase the likelihood of flash flooding. In a forest environment, the fungi increase canopy, and the canopy mechanically slows the water. Except in deciduous forests in the winter, there is no canopy, only mostly-bare branches. Although typically BC and northern Washington seem to be overpopulated with conifers so that may not apply in this case.
Fungally dominated soils have a greatly increased mineral and water availability due to the meddling of the fungi in soil chemistry. But if you dry out fungi enough, they become hydrophobic, like sphagnum does, and this can increase the likelihood of flash flooding. In a forest environment, the fungi increase canopy, and the canopy mechanically slows the water. Except in deciduous forests in the winter, there is no canopy, only mostly-bare branches. Although typically BC and northern Washington seem to be overpopulated with conifers so that may not apply in this case.