We have shown in previous work that the risk of subsynchronous resonance (SSR) between wind power plant with Type III wind turbines and a series-compensated transmission line is low when the wind turbines are operated in gridforming mode, instead of the standard grid-following mode. This paper explains the fundamental mechanism behind the improved damping characteristics by modeling the positive- and negative-sequence impedances of Type III wind turbines in gridforming control mode. It is discovered that the grid-forming control naturally acts against the negative resistance behavior of Type III wind turbines at subsynchronous frequencies that results from an interaction between the rotor-side converter current controller and negative slip. The developed sequence impedance models and improved damping behavior are verified using PSCAD simulations of a 2.5-MW Type III grid-forming wind turbine. The modeling predictions are also supported by experimentally measuring the sequence impedance response of a 2.5-MW Type III wind turbine in grid-forming control mode.