Poor Workmanship in Housing Construction 

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The backyard retaining wall is only 1m high and is made up of 9 inch red bricks. It failed or at the brink of failure. Why? 

If you look at the failed plane and the muddy site conditions, you probably know why.
Brick retaining wall is formed by cement bonding of bricks plus brick interlocking and self gravity. Cement mortar is the chief bonding agent to bind the bricks together. The cement mortar is a mixture of one part cement to three parts sand plus the water-cement ratio of 0.45 to 0.6. The sand cannot be too fine and too coarse, somewhere in between. Lower water-cement ratio has to be used if the sand is wet (bulking during raining). Therefore obviously, the mix is in suspect, likely too wet. 
The surface has to be clean before applying cement mortar. If there is a layer of clay slurry, there is no way the cement will bind the bricks. This is likely the main cause for this failure. 
The cement has to apply uniformly throughout the surface, but looking at the photograph, you will find some bricks very clean, no cement mortar at all! The gaps between many bricks are empty, thus the surface bonding area is effectively reduced. Design normally assumed full plane surface area, 9 inch brick + 1 inch mortar x length.  If you measure the brick size, probably the length of each brick ismay not be 9 inch. Try to measure the size, you will understand!
Uncontrolled site management, poor workmanship and non-standard materials are the main culprits.