It’s complex. Essentially our two systems were close enough to traverse with their technology and it was worth their while due to their systems instability.
The title “Three Body Problem” is referencing their systems three suns and their world’s unstable orbit.
It’s worth the read.
Within the Fermi Paradox is the Dark Forest Hypothesis, essentially: we can’t see other civilizations doing this, and the theory is that if they did they are wiped out immediately by other civilizations.
The Three Body Problem is a good novel exploring this.
It is akin to being invisible in a dangerous place and then turning on a flashlight and giving your position away to predators.
Tap for spoiler
It was using a high power radio transmission through the corona of the sun to act as a lens to magnify the signal, essentially turning our sun into a lighthouse. The shit that happens in the third book is mind melting in comparison. Dimensional attacks (shifting your dimension downwards) from 3D to 2D, by a potentially multidimensional entity, ironically unrelated to the original antagonists, and thus confirming the Dark Forest Hypothesis.