Riser wall corner joint has opened up, creating entry point for water. This joint needs to be covered with a piece of corner trim.
This riser wall appears to have been built with the top having a larger diameter than at the floor. The siding appears to have been cut to fit this condition. If that is the case, this is probably not a structural problem. If, however, the top has “enlarged” or bowed out after construction, it is a problem. Corrective measures are determined on a per project basis.