I believe the answer is that the turns are formed from complementary base pairing and cause separation of the RNA transcript and RNA polymerase.
A hairpin loop is an unpaired loop of messenger RNA that is created when an mRNA strand folds and forms base pairs with another section of the same strand. Intrinsic termination of transcription in E.coli involves the formation of an RNA hairpin in the nascent RNA. The hairpin plays a central role in the release of the transcript and polymerase at intrinsic termination sites on the DNA template.
