Understanding the origin of the tokamak thermal quench (TQ) remains a critical unresolved issue, particularly the causal link between initial localized temperature collapse and the final core energy collapse. This work presents a detailed experimental investigation of the TQ process on the J-TEXT tokamak, utilizing high-resolution diagnostics at multiple toroidal locations to observe disruptions with rotating precursor modes. We demonstrate that the initial localized temperature collapse near the q = 2 surface and the core temperature collapse are not independent events but are sequential phases of the continuous evolution of a cold structure (CS), thereby establishing a direct causal link between the two events. This CS originates near the inner separatrix of the pre-existing m/n = 2/1 magnetic island, and it is observed to be in-phase locked with the island. This CS is identified as a helical structure with a dominant m/n = 1/1 character, when it develops towards the core. The continuous radially inward and poloidal expansion of this structure drives an asymmetric temperature collapse. This provides a physical explanation to the increase of measured core electron temperature during TQ, which results from the rotation of remaining helical hot core. The observations are also applicable to auxiliary-heated plasmas on J-TEXT. The initial collapse is discussed, which is likely triggered by the excitation of 3/2 island. Additionally, a reduced model is developed to describe the magnetic topology and to simulate the temperature evolution. These findings indicate that the stochastic field during TQ originates from a localized region during precursors and evolves to the global stochasticity at the final energy loss.