I'm not sure why they did so in many cases, they didn't really elaborate much. Here are some quotes...
"There are, then, of the Old Testament, twenty-two books in number; for, as I have heard, it is handed down that this is the number of the letters among the Hebrews..." - St. Athanasius
"But it should be known that there are twenty-two canonical books, according to the Hebrew tradition; the same as the number of the letters of their alphabet." - Origen
"Observe, further, that there are two and twenty books of the Old Testament, one for each letter of the Hebrew tongue. For there are twenty-two letters of which five are double, and so they come to be twenty-seven. For the letters Caph, Mere, Nun, Pe, Sade are double. And thus the number of the books in this way is twenty-two, but is found to be twenty-seven because of the double character of five. For Ruth is joined on to Judges, and the Hebrews count them one book: the first and second books of Kings are counted one: and so are the third and fourth books of Kings: and also the first and second of Paraleipomena: and the first and second of Esdra. In this way, then, the books are collected together in four Pentateuchs and two others remain over, to form thus the canonical books." - St. John of Damascus
"I count therefore, twenty-two of the ancient books, corresponding to the number of the Hebrew letters." - St. Gregory the Theologian
