my thought was more a bad weather VFR circuit, flown at 500 to 600 ft agl (1500 ft on the qnh), yes indeed perhaps on the AP, unexpected IMC at the end of the downwind leg after having been in view of the runway all downwind and so feeling safe, perhaps expecting just a puf of cloud so waiting a second or two to find himself in solid IMC.
Fog and clouds tend to hang at the mountain edge
suddenly in IMC, so checking his map for the base leg heading (its a double base leg remember not 90 degrees)
CFIT