It happened to me for the first time last week. At least it could have been cell phone interference. My phone was on vibrate mode and in my trouser pocket, my recorder and preamp in my lower inside jacket pocket, probably only 6 inches apart from the phone.
However there is another possibility. The club I was in lies directly beneath Glasgow Central Railway Station. You can hear the trains trundling over before the band comes on. There must be a hell of a lot of signalling and telecommunications cables running along there too, athough you would think they'd be pretty well isolated?
I had cleaned all my contacts prior to the show so im sure that wasnt a problem.
My best bet is that it was the vibrating phone, i did receive a couple of emails during the show.....