According to the Geth memories shown by Legion in Mass Effect 3, we see that the Geth were shut down and attacked after they were created by the Quarians. The Geth responded by self-defense but what made the Quarians attack the Geth in the first place?
They feared, that the Geth would attack first. It was just an action out of fear. The Geth were fast expanding their intellect and that made the Quarians fear, they would rise against them, because they were stronger and more intelligent than the quarians, especially when they are near each other. The Quarians were shocked when the Geth started to ask Questions like a learning entity, like a living being. But they were designed as a VI which are networked to operate accurate. They tried to shut them down, but this failed and the Quarians began to kill the Geth. Just because of that, the Geth started to defend and this resulted in the Morning War.
So in short: The Quarians attacked the Geth because they feared them.
This is what i understood of what i have learned.
The Quarians first created the Geth to do dangerous tasks or mundane tasks kinda like how we use machines, to improve them the Quarians gave the Geth a network intelligence, the more Geth that was together the smarter they were because they could share information between each other.
Over time the Quarians created patches for the Geth Software keeping in accord to Citadel Conventions on developing Network AI, Tali described what they did as possible skirting the law but the idea was never to make an AI.
One day a Geth Platform approached it's overseer and began asking question on the nature of it's existence, this ment 2 things to the Quarians
The Geth were evolving to true AI status
If the Geth were self aware, the Quarians would be effectively using them as slaves
an order went out to all Quarian worlds to stop production of Geth and to shut down all active units but this failed, the Geth began to gather. though their Network Intelligence they began to understand that Quarians had tried to kill them so they revolted.
To some Quarians like Admiral Zaal'Koris vas Qwib-Qwib (can not keep a strait face with that) they saw the attempted shut down of all Geth as mass murder or genocide while other like Admiral Daro'Xen vas Moreh saw the Geth, even in their self aware state, as still tools of the Quarians like a hammer is to a blacksmith (not 100% on that line but i do remember Xen asking Shepard if he is mindful of a tool as it is used, it's hard not to think of Morrigan's face behind that mask). regardless of Quarian views the Geth drove them from their worlds and into exile.
In Mass Effect 3, the Admiralty Board voted 3-to-2 to start their war against the Geth to reclaim their homeworld, the votes were
- Admiral Han'Gerrel vas Neema -> For
- Admiral Daro'Xen vas Moreh -> For (only because it would allow her to experiment and possible bring Geth under Quarian control)
- Admiral Zaal'Koris vas Qwib-Qwib -> Against
- Admiral Tali'Zorah vas Normandy -> Against (due to her interactions with Legion saw peace is possible)
- Admiral Shala'Raan vas Tonbay -> For (she is known to take a neutral stance in the debate, not 100% what pushed her vote but given the other's views her's would've been the deciding vote)
Had Tali's father, Admiral Rael'Zorah still been alive, Shala'Raan vas Tonbay's vote would be unnecessary as Rael'Zorah would have voted for as he wanted to reclaim the homeworld to build Tali a house on it