fall out of ~は「（使用くせなどを）やめる」という意味のイディオム。the habit of speechは発話行為そのものを指しています。よってhad fallen out of the habit of speechは「言葉を話すことができない」と解釈できます。（Koyamamoto）
Vietnamese father and son found living in a treehouse for 40 years
A father and son who fled their village during the Vietnam War 40 years ago have apparently been discovered living in a treehouse deep in the jungle.
Ho Van Lang, 42, and his 82-year-old father had reportedly shunned contact with the outside world after his wife and mother of his two children was killed by a landmine
A local newspaper reported that they wore loincloths made of bark and used a homemade axe to chop down trees for firewood. They survived on corn that they had grown, plus fruits and cassava roots from the jungle.
Inside their treehouse home, five metres in the air, the pair kept a stash of arrows for hunting and knives for killing animals.
But there were poignant mementos of their previous life.
The father kept his soldier’s trousers neatly folded in a corner. Beside them was the little red coat his son was thought to have been wearing when they fled.
Vietnamese media tracked down another son of Mr Ho senior – a man named Ho Van Tri, who said that he discovered his father and brother 20 years ago, but had not been able to persuade them to return to modern society.
He said he brought more people to see them in the jungle to help bring them home – but they would hide quickly whenever they saw anyone approaching.
The pair were said to be barely able to communicate with outsiders, with Mr Ho junior only knowing a few words of the local dialect of the Cor ethnic minority group, while his father had fallen out of the habit of speech.
They were discovered by a party of local people who were travelling through deep forest in Tay Tra district of Quang Ngai province, in central Vietnam.