The idea of a 'Rogue Modron' originates from the Planescape Campaign Setting, specifically in The Planewalker's Handbook, on page 76.
Modrons are the clockwork caretakers of the gears of Mechanus, virtually unthinking in their strict hierarchical order. No beings’ minds are as focused on law, stability, repetition, and the security of harmonious regulation. But the multiverse isn’t perfect - not even in Mechanus - and
sometimes things go wrong. Sometimes a modron receives conflicting orders from two or more superiors, or is confronted with incontrovertible proof that all is not orderly. Sometimes a modron’s mind just snaps.
These circumstances create rogue modrons. When modrons go rogue, they lose
most of their special abilities and even the normal modron form that designates their position within the clockwork hierarchy. They find themselves cast out of Mechanus, the heart of law and the only world they
have ever known, and plunged into the cold, lonely, and (worst of all) chaotic multiverse. Rogue modrons have the forbidden and reprehensible glimmerings of self-awareness, the only thing that allows them to survive in this new sphere.
There's a lot more detail on them in that book, I'd suggest giving it a read. But the heart of it is that Modron's are creatures of pure order...and exposing them to too much disorder (especially within their own ranks) is not good for their minds.
The biggest takeaway here, though, is this: They are self-aware. Normal Modrons are not. A Rogue Modron is no longer part of the mechanical hierarchy that is Modron civilization. Also of note...they all look basically like Quadrones.
I would also note that 5E does have an extra way they can 'Go Rogue'
A modron unit sometimes becomes defective, either through natural decay or exposure to chaotic forces.
Couple that rule with The Great Modron March (mentioned on the same page of the Monster Manual), and you have a vast host of Modrons trucking around the Outer Planes. Some of which are very chaotic indeed. To give you a hook to work with...this sounds like a great opportunity to expose Modrons to 'chaotic energies' or 'proof that all is not orderly' and possibly cause large groups of them to go Rogue.