The zonal flow is of importance in the suppression of anomalous heat transport in fusion plasmas. Based on a gyrofluid model, it is found that the zonal flow generated in electron temperature gradient (ETG) turbulence can be enhanced in the regime with weak magnetic shear and steep temperature gradient. Then, a higher electron energy confinement state is predicted, which is characterized by the reduction of the anomaly of turbulent electron transport. In this work, the saturation mechanism of the enhanced zonal flow in weak shear ETG turbulence is investigated. The excitation of Kelvin-Helmholtz (KH) mode is proposed as a primary mechanism. Some considerable evidences of KH mode excitation in ETG turbulence are presented. It is found that the KH mode is weakly unstable for the observed enhanced zonal flow in simulations. The spatial Fourier spectral analysis shows the ETG turbulent fluctuation is dominated by longer wavelength modes with same spectral structure of KH mode. On the other hand, the time-frequency wavelet analysis of turbulent fluctuation shows the excitation of a low frequency fluctuation when the zonal flow grows up to some level. A visible frequency gap between the ETG mode and this low frequency fluctuation is observed. It may be deduced that a turbulence transition from ETG-dominated to KH-dominated fluctuation can occur in weak shear region of tokamak core plasmas.