Replacing a sill plate under an exterior door.

I am finishing the basement in the house we recently moved to and believed that I was in the home stretch - until...
I removed the last of the framing and insulation and found a section of severely rotted sill plate and rim(skirt?? ) board underneath our side door (which is in the laundry room). The damage is countained to a section of sill plate 5 feet long immediately under the door.
The foundation wall is out of the ground by about 20 inches on this side of our house. Thus my surprise at finding the water damage.
I am at a loss as to why we have this water damage. The only thought I have is that snow (we live in Canada) has drifted on the stairs and has rested against the brick and the transition from the foundation wall and the brick. I am guessing that this snow would melt and work its way into the house.
Would this type of action cause a rotted sill plate over time. Our house is about 25 years old and I understand that this side door was not used by the previoius owner... and presumably they did not clear the snow from the steps. The lock was seized when we moved in... the owners said they hadn't opend the door in over 5 years.

Any opinions you can provide would be helpful. I am planning to replace the damaged sill plate and ridge board but want to fix the cause first.

Thanks so much for your help!