The subsynchronous oscillations (SSO) caused by the dense integration of direct-drive permanent magnet synchronous generator (DPMSG) based wind farms have threatened the secure and stable operation of the local Hami power grid in Xinjiang Region, China. The DPMSG-involved SSO events show three new characteristics: random occurrence, time-varying frequency and amplitude, and wide involvement of all types of power electronics devices. To reveal the mechanism of the three new characteristics shown in the real SSO events, this paper develops the forced SSO (FSSO) theory and its method of use. Firstly, the generation and the time-varying performance of subsynchronous interharmonics (SSIH) from the DPMSG converters are analyzed based on the modulation theory. It is discovered that the SSIHs from the DPMSGs can be the disturbance source that excites the positive-damping subsynchronous mode to cause the FSSO. Further, the three occurrence conditions of the FSSO are proposed, and the three characteristics of the FSSO are revealed. It is concluded that the characteristics of the FSSO coincide with those of the real SSO events in the local Hami power grid. Finally, the theoretical analyses are validated by simulations in case studies.