Flow of water in a uniformly heated horizontal cylindrical duct induces a mixed convection phenomenon. For adapted coupling of the Reynolds and Rayleigh number values, an instability is reached for which fluctuations of great amplitude in the wall temperature occur sporadically. The aim of this paper is to analyze this thermal instability and to propose a physical mechanism which governs this kind of instability. Indeed, we show that this instability is the result of an exchange of the heat transfer mode from convective to diffusive in the thermal boundary layer, in the bottom of the cross section.