This is typically caused by an improper mortar mix during installation of the brick.  The problem this creates is that erosion of the mortar may occur over time with normal water action at the siding.  You will see this most prevalent at the window sills, fireplace chimneys, and at the lower exterior walls under a roof valley discharge or adjacent to a patio surface where water is caused to frequently wash over the mortar.  If the mortar erodes away it can leave holes that allow excessive water entry behind the brick and can increase the potential for water entry to the building.  If the mortar is soft it can mean poor workmanship in installation and may indicate that other common installation defects exist including excessive mortar slag behind the siding blocking weep holes.  This combined with erosion in extreme cases can create water entry problems.

I checked with Jorge Alverez at Masonry Solutions, a local masonry contractor, and this is his recommendation:

The proven corrective action is to tuck-point the failing mortar joints, is the process of grinding the mortar joints approximately 1" and replace it with new.