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Why did the Jin dynasty turn against the Song dynasty after the fall of the Liao empire?

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Map of eastern Asia highlighting Manchuria in northeastern China, which includes the provinces of Heilongjiang, Jilin, Liaoning, and part of the Inner Mongolia Autonomous Region. The highlighted area borders Russia, Mongolia, and North Korea. — Manchuria The historical region of Manchuria shown with the boundaries of the modern-day Chinese provinces in its place as well as the portion of the Inner Mongolia Autonomous Region that is often considered part of Manchuria.

The Jin dynasty, established by the Jurchen tribes of Manchuria, turned against the Song dynasty because the Jurchen had developed their own imperial ambitions after successfully overthrowing their former overlords, the Liao empire.12

Initially, the Jurchen and the Song were allies against the Liao. However, once the Liao dynasty was destroyed in 1125, the Jurchen were no longer constrained by that conflict and were free to devote their full military attention to conquering the remaining Chinese states, including the Song. Having developed a powerful military force, the Jin sought to expand their rule over North China, leading them to launch an invasion against the Song that resulted in the capture of the capital, Kaifeng, in 1127.132