Discharge activity associated with water droplets on a polymer insulator surface is thought to be the main mechanism of the early stage of hydrophobicity loss. The mutual positions and dynamics of the droplets affect the probability and the cause of the partial discharges. Therefore, this works aims to observe the dynamic behavior of droplets and the associated dynamics of surface discharge during the process of liquid runoff on silicon rubber surface in the presence of the tangential AC electric field. The research is carried out using an electrode system of two parallel plate electrodes with a sample between them, where NaC1 solution flows down on the surface of the sample in a form of separate droplets. Two ultra-sensitive cameras record droplet positions and weak partial discharges simultaneously, which makes it possible to know exactly what event caused each discharge. Two stages in the behavior of discharge activity are distinguished during the loss of hydrophobicity. At first, droplet mergings and multi-drop runoffs are observed. In the second stage, water channels form and break-up after the droplet merging. After the loss of hydrophobicity, discharge light is observed continuously at the ends of the formed conducting path. The revealed behavior of the discharge activity helps to clarify the scenario of the hydrophobicity loss. The accumulated intensity of the discharge light during accelerated aging of the sample can be further investigated as a quantitative parameter to evaluate how resistant material is to the loss of hydrophobicity under the action of surface partial discharges.