Experiments were conducted to reveal the origins of measured inefficiencies in low-momentum jet diffusion flames in crossflow. Natural gas flames were established at the exit of a burner tube mounted vertically in a crossflow of air in a wind tunnel, a configuration that is relevant to gas flaring in the atmosphere. At low momentum flux ratios, the flame exists in a wake-stabilized mode and exhibits combustion inefficiencies primarily in the form of emitted unburned fuel. Measurements with a fast flame ionization detector were used to determine the path of the unburned fuel being emitted from the flame, Analysis of the hydrocarbon concentration time series data show that hydrocarbons are ejected from the underside of the flame in a highly intermittent process. A mechanism is proposed in which the mean flow induced by the standing vortex that exists on the leeward side of the stack transports and stretches the ring vortices from the upper shear layer and ejects them on the underside of the flame.