Once a joint recedes past the brick face, storm water stops running off the wall and starts driving into it. That is the mechanism here. A vertical shower does little damage to sound masonry; the same rain pushed sideways at forty miles an hour into an open joint is a different event entirely, and DFW supplies several of those a year.
The prep decides everything: joints ground out until sound material is reached, then repacked in layers and tooled to the original profile. A smear over crumbling mortar survives one season of Texas sun and fails in the next storm. Once fresh joints are in, sealing them is worth pricing — waterproofing with a vapor-permeable sealer sheds driving rain while letting the masonry breathe, and it is cheapest while we are already on the roof.