Speculation for everyone :)But I think it really doesn't make any sense to dump all your civilians on a planet in the middle of the reaper war...undefended. They would be safer on their three liveships...just hold them back in battle (three ships won't make a difference anyway). I mean, at least if they stay on ships they have a chance of running away if the reapers show up in force. Because if the reapers show up on Rannoch, you've just lost ALL your civilians.I think they finally had the advantage with Xen's weapon and used it. And that is the reason they attacked.
Pretty sure we've been over the logistics argument before. They can't divide the fleet, protect and feed everyone at the same time. They can't do the work they need to be doing in this war if they can't split up. Even before the war, the fleet depended on external infrastructure for basic survival. With the Reapers here, this infrastructure is being rapidly destroyed.
The point of taking
Rannoch wasn't to 100% guarantee the safety of the civilians. Nowhere is safe in a Reaper invasion. But the thing is, when your cargo hold is physically full of housing for hundreds of civilians in prison cell-sized cubicles, and you need to stay close enough to a liveship for daily shipments of food to keep them all fed (per the codex entry on liveships), your ship is not available to haul war materiel or soldiers around as Hackett needs. By offloading their civilians, they make their ships available to aid the war effort. They enable themselves to split up instead of being clumped together in a single mass which takes days to traverse a relay and, as we see through the Geth blockade, can easily be corralled in a single system. They also guarantee that they aren't needlessly hauling hundreds of civilians on every ship into the fight any time they engage Reapers in combat.
As we see from the map description, the civilians are offloaded with a token force from the patrol fleet for protection. They're hedging their survival on the Crucible being completed (with their assistance) like everyone else. They could have forgotten about aiding the other species against the reapers and attempted to hide in space (this was Koris' alternative to war with the Geth), but they instead chose the route which let them "throw everything they had" at the Reapers.
Had there been better communication between both the Quarians and the Geth, the situation may have been averted completely, but that unfortunately was not the case. Honestly, the Quarians did exactly what I'd have wanted them to do: arm up to the best of their ability, take minimal steps to ensure the continuity of their species, and commit every resource at their disposal to the war with the Reapers. If only others had followed their example. The council races (who already had the benefit of large, widely-dispersed populations and industrial bases) ought to have been churning out dreadnoughts as fast as they could mine the metal.