I've reported a similar bug to the OP a ways back.
It would be interesting to me to know if the cities being founded without the expenditure of a pioneer were at 30 population instead of 3?
With regards to rushing I think it was very improbable in my case (using cheats you could check the saves I posted to see if the faction expended an appropriate amount of gildar but I'm pretty sure that it didn't...). Turn 30 on beginner, I think at epic pacing. Given that Magnar recruited a champion I'm not sure where he got the cash from. Further that pioneer went on to found several cities before it disengaged from the army and got gobbled by a monster. Maybe there was enough money for one rush but certainly not for another 6-8 turns later, and another after another 6-8 turns.
As far as I can work out the trigger is having a pioneer grouped with other units in an army, although it is possible that one of these units needs to be a champion.
Frogboy didn't understand the problem. In another post he claimed to be skeptical since he could not reproduce this. Maybe he could have loaded the saves. Should rename him Ostrich.