Next Thing One Knows Meaning

(idiomatic, colloquial) suddenly, out of the blue.

Example: 1990 July 22, “2 Roofers Hit by Lightning, Man Critically Injured”, South Florida Sun - Sentinel:
  ([Milton Dosantos]) just said they were on the roof and the next thing they knew, they were flat on their backs
1998, Al Rapaport, Buddhism in America
  He had three fingers on one hand because one of them had been shot off, and he'd sit there doing this and next thing you know, out would come this three-dimensional folded cage or a box or cranes or snails. Really remarkable.