I had one race (The snathi) do this for the first time ever to me the other day, I was suprised and checked to see if they had conquered the planet (far more common for me) off the terran. Nope, they had colonized another snathi planet (some pq 7 peice of crap).
Be careful though. Sometimes minors are well placed with the race that shares thier fuel lines, I had the carinoids and the drengin in similar space and the drengin life support lines were being used by the carinoids (Each minor is attached to a major in this way. It's why the play stupidly sometimes or just seem to stop moving shortly after some major dies out) Well for some reason the carinoids and drengin both went to war with the torians and the Carinoids had 17 plants and 2 nice sized fleets when the torians turned over thier remnant empire to the terrans. They then siezed two iconian worlds... Minors can really get rolling and when I checked the carinoids were considered more powerful then 3 major races, I supplied them with 15000 bc and 12 or so warships (I hate the torians and saw it as an opprotunity to screw them) but when the war was over they had only two of my warships left out of 30 or so warships I could count in my line of sight. Another game had the Paulos up to 19 worlds or something when they decided to go to war with me and I snatched them all up. Play a game long enough and these minors will get going if they have good placement and early luck, I've noticed U.P. meeting where everyone trades techs seems to even the playing field out for them dangerously. Either it's that event or the akilians steal my techs cuz they had industrial sector when I was the only who had it or was even close to having it.