The Yenisei Kyrgyz played a significant role in the collapse of the Uyghur Empire. In the mid-9th century, specifically in 840 AD, the Kyrgyz successfully attacked and destroyed the capital of the Uyghur Empire, Karabalghasun. This attack led to the death of the Uyghur chagan (leader) and contributed to the empire's fall. This event triggered a significant migration of Uyghurs from Mongolia into Turfan and other regions. The destruction of the Uyghur state by the Kyrgyz is seen as a crucial factor in the empire's disintegration.
For more detailed information, you can refer to the Wikipedia page on Yenisei Kyrgyz and this publication.