This is how Furno explains Von Ness' origins goes in A Canterlot Wedding Part 2.
[Rainbow Dash looks confused]
Rainbow Dash: Who's Von Ness?
Furno: It happened back when Stormer was a rookie.
[His face shows a screen which flashbacks to the New Stellac mission]
Furno: (narrating) It was supposed to be a routine hero response to a burglary in progress. So team leader Thresher took two rookies along with him. And it all seemed routine until they found themselves against this hulking drone, packing enough plutonium cryptide canisters to level half the city.
[The drone throws a bomb. Thresher and Stormer fire at the drone, which turns its attention to them]
Furno: (narrating) So they fired to draw its attention while Thresher, protecting the rookies, made a run at the drone.
[Thresher runs at the drone and avoids two blasts from it before getting hit and landing on the ground.
Furno: (narrating) Thresher took a bad hit, so Stormer and the other rookie--they're on their own.
[Stormer takes Thresher to the dropship they came on]
Stormer: Von Ness. It's up to us to stop that drone before it hurts any civilians.
Von Ness: No. We can't. We have to wait for backup.
Stormer: By then, it could be too late.
Von Ness: I don't care. I'm staying here.
[Stormer and Von Ness glare at each other then Stormer looks back at the drone coming towards them]
Stormer: Okay, you cover Thresher, and I'll draw the drone away from you.
[Stormer turns around and runs and fires at the drone]
Furno: (narrating) Stormer decided to take on the drone himself while Von Ness covered Thresher.
[The drone attempts to stomp Stormer, but Stormer ducks out of the way and prepares to fire at it when he hears the sounds of engines and turns around to see their dropship taking off]
Furno: (narrating) But Von Ness had other plans.
Stormer: Von Ness! [groans]
Furno: (narrating) Can you believe he was just going to leave his injured team leader and Stormer?
[Stormer jumps and pounds his weapon the window]
Stormer: Bring it down. now!
Von Ness: No! you'll get us all killed.
[Von Ness flies off and drops Stormer]
Furno: (narrating) It didn't look good for Stormer or New Stellac City. Check out what Stormer does next: Maybe the coolest move in Hero Factory history. First, Stormer knocked out the drone's visual scanning.
[Stormer throws a punch]
Furno: (narrating) Then, landing on its back, he cross circuited the power supplies, totally crippling the drone.
[Stormer lands with a thud as the Drone falls to the ground]
Furno: (narrating) Stormer saved the city and his team leader. But he never came to terms with the idea of Von Ness going rogue.
[The flashback ends]