Battle of Autas took place because the Muslims had to defend themselves against the league tribes who formed an alliance together to attack the Muslim community, the battle of Hunayn. It was a war of self-defence.
Please also read the following article on what happened to the captive women, to get a better understanding of the incident: “What Happened To The Captive Women In Awtas Incident?”
The Awtas (Autas) incident took place as a result of the enemy, Hawazin and Thaqif (Thakefites) using this area for attacks against the Muslims. This is reported in many authentic reports that they used Awtas area for military purposes.
“Abu Sa’id al-Khudri reported that at the Battle of Hanain Allah’s Messenger (p) sent an army to Autas and encountered the enemy and fought with them. …” (Sahih Muslim, Book 008, Number 3432)
Kitab Tabaqat Al-Kabir – Ibn Sa’d:
“THE GHAZWAH OF THE APOSTLE OF ALLAH AGAINST HUNAYN, AND THE SAME WAS THE GHAZWAH OF HAWAZIN
… They (narrators) said: When the Apostle of Allah conquered Makkah, the notables of the Hawazin and the Thaqif moved from one side to the other. They assembled and rose in rebellion. Malik Ibn Awf al-Nasri had brought them together and he was then thirty years old. He ordered them and they brought with them their wealth, their women folk and their children. They mobilized at AWTAS and the supporters were coming to them. They agreed on marching against the Apostle of Allah.” 
“The Prophet had been staying in Mecca during the year of its conquest for only a forth-night when [he received the news that the clans of [Hawazin and Thaqif [were marching against Mecca and] had already encamped at Hunayn intending to fight him. Hunayn is a valley next to Dhu al-Majaz. Both the aforementioned tribes had assembled before [their match] after hearing about the Messenger of God’s departure from Medina, thinking that he was intending [to invade] them. When he learned that he had occupied Mecca, Hawazin marched against him [to Mecca] with their women, children, and possessions. Their leader was Mailk b. Awf from the Banu Nasr. [The clans of] thaqif joined forces with them and encamped at Hunayn intending to fight the Prophet. When the Prophet, still in Mecca, was informed about them he decided to march against them. He met them at Hunayn, and God, the Great and Mighty, inflicted defeat on them. God has mentioned this battle in the Qur’an. Since they had marched with their women, children, and flocks, God granted them as booty to His Messenger, who divided the spoils among those Quraysh who had [recently] embraced Islam. …
When Malik [b. Awf] decided to march against the Messenger of God, he took with his men their possessions, wives, and children. After making camp at AWTAS, the men gathered around him. Among them was Durayd b. al-Simmah, who was carried in a Howdah [Shijar]. As soon as he had encamped he inquired what valley they were in. When he was told that it was Awtas, he said: ‘What a wonderful place for CAVALRY! Neither a rugged hill, nor a soft lowland full of dust. …” 
“The Enemy’s March and their Encampment at Awtas
When Malik bin ‘Awf — the general leader — decided to march and fight the Muslims, he made his country people take their wealth, women and children with them to Awtas — which is a valley in Hawazin land and is quite near Hunain. It differs from Hunain in its being adjacent to Dhi-Al-Majaz which is around ten miles from Makkah in ‘Arafat’s direction. [Fath Al-Bari 8/27,42] 
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 Kitab Tabaqat Al-Kabir, By Ibn Sa’d, volume 2, page 185
 The History of al-Tabari: The Last Years of the Prophet – [Translator: Ismail K. Poonawala], volume 9, page 2 – 4
 Sealed Nectar (‘Ar-Raheeq Al-Makhtum’), By Safiur-Rahman Al-Mubarakpuri, page 213