These last years, convoying covered trucks by railroad highways had improved our manner to transport freight supplies. However, this kind of transportation generate truck’s tarpaulin breakout, forcing the train to stop until the problem is fixed. Instabilities induced by the separation of the flow facing the truck are mainly responsible for this phenomena. In such a context, the present paper focuses on the flow surrounding a forward facing step whose separated region is governed by complex dynamics which reproduce the aerodynamics involve on the truck’s flapping tarpaulin. The dynamics of the low-frequency large-scale flapping motion and its connection with the dynamics of the flow just upstream of the step are investigated. The connection between the two recirculation regions and particularly more the influence of the upstream bubble on the downstream bubble shear layer is shown. The study suggests the importance of these mechanisms which can be key features for further flow control purposes.