No I don’t think parking on the diagonal is what the City of Seattle Streets Department had in mind here. This was observed a few days ago in the neighborhood.
The street is a bit steep and the driver (or car) backed over the curb and stopped up against the tree. No snow or ice the night before to blame things on. I am sure there is a real logical reason for parking this way.
This is a time honored way however in some areas of keeping a vehicle from rolling down the hill. Many people I know carry a large block of wood in the truck if the brakes don’t work all that well or they want to be safe (take a look at tank trucks and the such when they are unloading, many will have chocks for the wheels even when they are on level ground).
I hope the tree was not damaged.