To me seems it was the created or integrated Protean Reaper tasked with proving its value by starting the reaping cycle. I say this as Soveriegn nails that Prothean superiority and arrogant attitude. It could also be an interpretation of how a prothean sect failed to control Reapers and instead joined up with their beliefs or were “corrupted by the code”. Perhaps an Illusive Man parallel.

Could also contribute to how Reaper-Saren had an overload perhaps? In its arrogance it diverted too much energy trying to do many different tasks, including trying to stop Shepard and in the moment it was defeated it couldn’t handle the concept of losing to an inferior being which caused a logic error and subsequent crash.

It also played into the “God Complex” with the Geth, which seems very on brand with Protheans

Considering how they are cloned, it could have been another Prothean sect that was big on biosynthetic augmentation and attempted to survive the Reapers by trying to “hijack” a relay and escape to the galatic core and there finding the husk of some other cycle’s attempt at constructing the Crucible but failed. Not knowing that by them integrating “Reaper-based AI” code that it would corrupt them and over time lose themselves to Harbinger’s Will.

I say this as the Collectors only have one ship and how easy it is for Harbinger to interface with the augmented Collector General and Collectors. Interesting in how the Collector general seems confused once harbinger releases control - could also be that the general came to after being mentally dominated for who knows how long.

interesting how quick Harbinger is able to drop the charade of the starchild when it dismisses Shepard when they refuse on the Crucible in a similar manner to how it abandons the Collector General.

It is an interesting parallel to Cerberus and Collectors as well with them trying to integrate flesh and machine with Shepard, Project Overlord, not knowing the books, but I recall someone named Greyson as well with another Cerburus incident and also with attempting to pass through the Omega Relay and using any (advanced tech that in all likelihood has Reaper-Tech integrated) Collector Technology they find - although intended less for survival and more for dominance