Why did they bolt away?

This sound ridiculous! Why did the greatest army at that time on earth unable to crush tiny Israel in 66ce. 

They bolted from them.

This is one of the most unanswered events in the history of mankind. Imagine the greatest army on earth today- the US unable to crush a nation like Seychelles. That would be the most unheard disaster in the history of the United States. I am very sure that the President at that time will be impeached or be forced to resign. There will be rancor everywhere and other nations especially the rogue countries will turn the United States into a mockery pad.

What led to the most catastrophic events in the history of the Roman army?

The Roman has directly or indirectly asserted their authority on Israel especially in Jerusalem. They ruled the countries by means of proxies who collected taxes from the citizens and remit same to Rome. However, from 63Ce, a chain of events lead to a showdown. 

Ultimately, the combination of financial exploitation through taxes, Rome's contempt and intentional disregards for Judaism, and the unabashed favouritism that the Romans extended to gentiles( which is an abomination) living in Israel brought about the revolt.

A group of people called the Zealots rose up and try to defy the continuous harassment of the Romans. Their modus operandi is to fight with all means so as to attain political and religious liberty. They fought and burn roman soldiers wherever they were found in Israel. Florus in 66ce the last Roman procurator stole vast quantities of silver from the Temple. He desecrated the temple of God in Jerusalem. According to the Jews, the temple is not only the symbol of worship among the Jews but is a sign of God 'stool' with them.

Florus fled Jerusalem and the garrison in Jerusalem was crushed and  other rebellions broke out all over Judea.

 This victory gave the Jews the needed boost to conclude that the war against the Romans was winnable

.Cestius Gallus, the Roman ruler in neighboring Syria, sent in a larger force of soldiers. These forces include war-hardened soldiers. Their aim was to breach the wall of Jerusalem and crush the Zealots. They came with the belief that as Romans that they will crush the insurgents in no time. The stage was set for one of the finest soldiers at that time-Cestus Gallus to rekindle and reassert the dominance of the Roman Empire in Israel. The Roman never knew what awaits me.

The Zealots and their supporters have grown in the belief that the recent victory against the Romans will inspire them to victory. The Zealots supporters were growing in leaps and bounds

Though the  Romans started well as they took large areas of the north of Judaea and Galilee that had revolted without much effort. The Romans spread themselves too thin, The Zealots capitalized on the fact that the Roman was fighting in many fronts however, they ambushed and defeated the mighty Romans at the battle of Ben-Horon.

This was a shocker. A band of ill-equipped though motivated warriors defeated the mighty Romans under the guidance of Rome brilliant general. This was a bitter pill to swallow.

Their pride and honor vanished temporarily with the defeat. This emboldens the Jews to believe that God was with them and they celebrated it for days. Though the Romans came back with full force and crush, destroy and burn down Jerusalem in 70ce but this defeat in 66ce will go down as one of the most unsolved histories of world affairs.