I'm about half-way through the pilot. It's decent so far. Anyways, as far as those asking why they traveled to the time period they traveled to, it was never explicitly stated, but it was said that scientists discover an anomalous "fracture in time and space", which leads me to believe that this time period is the only point which they can travel to. They sent a probe through the worm-hole or whatever, but they didn't know where it ended up until they went through themselves and found it on Terra Nova.
Of course, then there's still the plot hole of how everyone who time travels remains on the same timeline; everyone who travels through would create an alternate universe, as they are traveling to a time before anyone else went through.