what makes gatso' randomly flash?

[Glaucus]

Was on the way back. Gloucester road opposite the tesco garage if anyone knows it. No other traffic about. I was doing 29-30. went past then after I had gone past and like 20meters after the white lines on the road. the gatso flashed.
Nothing else about. So what set it off?

 

[Glaucus]

Firstly I don't pay for tickets that are incorrectly issued, surely it costs the Police money to process such tickets when they inevitably get disputed?

A lot of people don't dispute.

So I might have to dispute a ticket wasting my time .