This phenomenon is normally caused by moisture being drawn from the surrounding soil. This is usually caused by the stucco coming in contact with soil and extended exposure to moisture, which allows the salts and minerals contained in your soil to “wick up” and migrate to the surface. This will eventually deteriorate and/or cause delamination of the stucco from the wall.
The most common causes are: water from sprinklers/lawn irrigation systems continually soaking the wall, followed by improper site drainage (water draining toward the wall). Moisture in the soil or water sitting at the base of the wall travels up the wall in a wicking action; due to a lack of Weep Screed at the bottom of your stucco walls.  This is frequently seen in homes built during and prior to the early 1980s construction boom.
This issue of “wicking” was addressed in our San Diego Building Codes in the early 1990s and isn’t seen on homes built after FHA screed was included in our San Diego building codes.  Unfortunately, we still see it on new newer homes in the county/rural areas of San Diego.  This is a straightforward issue to remediate, but it is labor intense and really requires an on-site inspection so we’re not just guessing.
The best part, our estimates are FREE, so call us at (619) 561-7429 and ask us for an estimate on your stucco repairs.  You can also use our Contact Form to request your estimate, and send us pictures.