[POTENTIAL SPOILERS]
No one other than the writers truly know yet, that's kind of the point. The marker is still a mysterious creation, especially the 'black marker' that was discovered on earth but went missing hundreds of years before the events of Dead Space. The 'red markers' we see in the games are actually man made copies of it. I'm assuming the outbreak reoccurred on the sprawl because of the Ishimura being salvaged. It says in one of the logs that there was fleshy masses all over the Ishimura that 'became active' as they approached the red marker on the sprawl. So as it docked they likely came back to life and invaded the sprawl, which makes sense as it was docked relatively close to the civilian district where the game begins.
It turns out the black marker actually had properties that STOPPED the necromorphs from approaching, it effectively killed them from a huge radius. The red markers also apparently give off this 'field' too but to a much, much smaller extent. The red marker on the planet in Dead Space 1 had its 'field' amplified by the platform it was placed on (this is outlined in a graphic novel), hence why the hive mind and necromorphs became inactive the moment you put the marker onto the platform. I can only assume that there were necromorphs 'trapped' on the planet in Dead Space 1, dormant due to the red marker on its plinth. The moment the unitologists on the Ishimura moved it, they came back to life. The question remains though, how did they get there? Why were they there? Did humans hundreds of years before hand copy the black marker to create the red marker from DS1, then use the code on it to create the necromorphs? Then realizing their mistake they built the plinth to trap them, dormant on that planet perhaps?
One thing that puzzles me is, the writing on the marker is apparently the genetic code for the virus that causes the necromorph infection, yet the black marker, the original, was clearly designed to repel them. Perhaps the necromorphs were originally an intergalactic biological weapon, used by a very advanced race before the existance of man. The black marker allowed them to create necromorphs, but in turn protected them from their creation.
Dunno, lots to it, lots of back story in graphic novels and films based in the Dead Space universe.
