大般涅槃經義記 8.165

8.165

言「因因」者牒第二門，「即是智慧」斥其體相，觀因緣智依前因緣，從因起因故曰因因。

The words "cause-of-cause" take up the second gate, and "is wisdom" identifies its substantial characteristic: the wisdom contemplating dependent origination relies on the preceding dependent origination, and because a cause arises from a cause, it is called a cause-of-cause.