Thanos Thanos:
Has that area been overly-logged and clear-cut over the last century? I know it's very hot and arid there but if most of the old forest got wiped out then the soil will have long been left incapable of growing the root-systems necessary to make the area less likely to get ravaged by flash flooding.
It's farmland and has been for over a century, so it's safe to say some trees have been cleared since then. And there used to be a shallow lake in the area called Sumas Lake which was drained in the 1920's.
The fires along he Coquihalla explain why the mudslides happened. The heavy rains were compounded by the fact that the weather system was a Pineapple Express; one that brings a stupid amount of rain as well as warm trade winds from Hawaii. Couple that with the higher temps causing a snow melt at higher elevations, and hence the severe flooding.