Most concrete retaining walls that are in your yard fail due to three reasons:
- Lack of Engineering
- Improper footings
- No rebar
- Lack of drainage
Older walls, especially those around 4’ or less in height, were built as “gravity walls”, which means they had little if any footings and were designed to stay up based upon their mass and weight. This will often be sufficient for many years, but eventually they start to lean and at some point fail due to movement of soil behind them–often related to lack of drainage. 
This is an example of a typical leaning gravity retaining wall. All the trees and vegetation behind the wall did not help matters, as roots and their expansion helped push this wall down over time.
