Solid-state electrolytes have the potential to stabilize lithium metal anodes, which hold the promise to increase the energy density of lithium-ion batteries. However, lithium metal dendrites that occur locally at the solid-solid interface plague the solid-state lithium metal cells during charging, limiting the cycle life, and rendering high safety risks in practical applications. While multiple explanations have been proposed, understandings of the dynamics preceding and causing the metal penetration in solid electrolytes are still not conclusive. Here, by testing many highly consistent Ta-doped Li7La3Zr2O12 (LLZTO) samples and utilizing an improved operando technique on symmetrical cells, we observed a consistent and statistically significant trend of current-dependent dendrite initiation time, which coincides with the Sand's time scaling in liquid electrolytes. This new understanding offers an electrochemical origin of dendritic initiation but also provides future insight into optimal all-solid-state battery design.