What could be the reason an ATX PSU triggers OVP?
Are you testing under load? This may well just be a characteristic behaviour of your PSU and not a fault per-se.

When operating under normal load switch mode power supplies operate in "continuous conduction mode". In this mode the output voltage is determined mostly by the input voltage and the duty cycle with load current only having a relatilvey minor impact (with ideal components it would have no impact), so the drive controller only has to make relatively minor adjustments to account for varying load and input voltage.

However under very light or no load switchmode converters that use diodes (which is most of them) enter what is known as discontinuous mode. In this mode output voltage is strongly linked to load current and far more radical changes in duty cycle are needed to compensate. 

This is especially problematic for power supplies that try to derive multiple output rails from different taps on one controller/transformer as the duty cycle needed for one output may not be the same as for other outputs.

The result is many switchmode power supplies, particularly older designs or designs with multiple outputs, have a minimum load specification below which they will not operate properly.