Yes, lack of drainage is one of the main causes of failures of retaining walls in my experience also, and understandably as the amount of soil pushing agains th

Yes, lack of drainage is one of the main causes of failures of retaining walls in my experience also, and understandably as the amount of soil pushing agains the wall increases its weight when wet, often more than 20% and water is also a good lubricant to allow for slides to happen.