This goes alllllll the way back to the original sin: illegally letting contractors, not agents or intelligence officers but outside hires, access the 702 database without a warrant or oversight. This is where this scandal started, with 44's administration doing "business as usual" (business likely learned from 43's administration) by illegally surveilling their political opponents through private contractors (think Fusion GPS, think Halper) who then leak the illegally gained information to media cut outs to create scandals and smears from smoke.
If Horowitz is moving here, then that's a GREAT sign.