For all we know perhaps the captain was flying an ad hoc visual maneuver attempting to avoid known (to him) areas "unfriendly" to overflight.
Possible... But not a good idea in the prevailing conditions.
A circling manoeuvre is a visual segment of an instrument approach, not a 'visual approach'. The in-flight visibility, maximum distance from the airfield and operating altitudes are all exactly specified, as BOAC has pointed out. As are the actions to be taken if you lose your reference or reach any of the other limitations.
If you're not completely clear about what you're attempting, it's possible to get caught by doing something half-instrument, half-visual and mostly dangerous. Minima are there for a reason.