The way I understand it, it's entirely related to fighting. Orks get bigger as they experience more fights, and as they get bigger and stronger, they also get better at fighting and prosecuting warfare. Thus, the more intense battles an ork is subjected to, the faster they'll develop.
This means that, in the hypothetical case of a single ork spore on an uninhabited world, it'd first spread the orkoid ecosystem - fungi, then squigs, then grots to manage those squigs and build primitive mushroom farming villages. Then the first ork boyz pop up, and immediately take over whatever grot village they're nearest to. As the orkoid ecosystem develops and spreads, more and more orks develop and begin to form into primitive warbands under whatever ork is the biggest and thus fightiest, and as this happens multiple times the ork warbands begin to fight each other. This is what most feral orks do, only they usually have other gits to attack on the planet as well.
As the orks continue to fight each other, eventually there will come a chieftain big and smart enough to be unusually successful in his campaigns, enough that he and his boyz start growing big enough to start thinking of more advanced concepts of warfare than just sticks and stones. Orks advance fastest when they're fighting strong opponents, so as the first warboss and his boyz become wildly successful with whatever invention they make, the other orks they fight soon start to develop to the same level. Soon the whole planet of orks has reached what's basically an industrialized warfare level of tech, where the same thing happens as in the feral stage - tech being mostly equal between everyone, various warbosses form up into the rough equivalents of modern-day nations and war with each other. Depending on how intense this warfare is and how long it takes for the right circumstances to come about, it takes a varying amount of time for this next part to happen, but eventually it does. Eventually, one particular warboss gets successful enough that he conquers the whole planet, and unites all the orks in it under his banner. Of course, this means they've run out of things to fight, so they have to go looking for more. So the orks either get lucky and hijack an existing ship/space hulk, or have to develop space flight on their own a la Kerbal Space Program. If this takes too long, the orks get bored with nothing to fight and the warboss loses his control over them, causing the whole lot of them to splinter back into the various nation-states, and the process starts over from there.