That's... an odd reason. There are plenty of stars out there, unless the aliens started out right next door (like in Alpha Centauri) there's not much reason to go after our star.
I haven't read the books...
This is where the aliens are (a trinary system). It still takes them 400 years to get to Earth and so they are trying to stifle Earth's technological advancements because 1) we know they are coming 2) our technological growth is faster than them (this is partially explained due to different biological and environmental factors. The aliens can't lie to one another and have environmental factors that frequently wipe out or pause their technological advancements). The aliens in question are supposed to be only a few hundred years (max) ahead of us technologically (or smaller than the difference in time that it takes them to get here)
They did.
:D
Strongly recommended reading.
The major bit of artistic license is that their Alpha Centauri is way more broken than ours; the real one isn’t all that badly behaved.
It’s implied that the whole situation is very unusual; interstellar invasions don’t happen as a rule and they’re only doing it because their star(s) is basically broken.