Certified GKH Gardening Expert
Possibly inadequate water and/or nutrient deficiency. Also, pictured are extreme heading cuts that were made for crown reduction. The resultant regrowth demands on the plant are very stressful, especially if plant nutrition is compromised and there are competitive plantings or especially high demand lawns and way too frequent watering schedule. Also to consider, heavy aphid infestions can further stress the plants.
Some people think you can't hurt oleanders, but in fact you can. Light or moderate pruning, no more than 25% reduction, use of slow release complete fertilizer two or three times a year, and a weekly or every two weeks deep watering is all they need.
From the photo it appears that the plants have become over-mature and overgrown for the site. They were cut back extremely to try and control the size. There is lawn on the other side of the wall that shouldn't be a big factor but that depends on the depth of the wall footing. If you have lawn on your side also, then that can be too competitive for nutrients and detrimental for the health of the oleanders.
You may want to consider removal and replacement with more suitable sized plantings. Some oleander varieties like yours get huge. And it's a shame to always have to shear and reduce and lose the beautiful flowering that occurs on the outer foliar crown.