While the player DKs are by and large technically “cannon fodder” DKs mass-produced right around the beginning of the Wrath timeline, several of the Ebon Knight characters were killed and risen in the Third War, like Thassarian (killed during Arthas’ expedition in Northrend) and Koltira (killed during the invasion of Quel’thalas), and both of those characters have the trademark blue lichfire eyes that often distinguishes a 2nd Gen DK (like Rivendare) from a 3rd Gen (Acherus) DK. Or at least Koltira does; Thassarian’s official artists seem a little confused and sometimes he has it and sometimes he doesn’t. But either way, they have the traits of the later generation and were raised very early on, so you can assume there must be more like them.
So I guess, in short, the answer would be circumstantial. Breaking free on your own just isn’t a thing that happens. Like a lot of the NPCs, they could have been an older DK that was present at Acherus for whatever reason (this is what I RP), or maybe they could have been stronger-willed and managed to break free in Icecrown in that brief minute before Bolvar took over. There isn’t a whole lot of info on what happened there, but considering a moment of weakness on Arthas’ part is what let Sylvanas and her forsaken break free, actually dying would be a big enough power gap to allow something to get out.
I don’t think everyone needs to be restricted to Fodder Knights for their character, but there would definitely need to be a good reason for how they got there, because most aren’t so lucky.