As with many historical events there are usually multiple causes and different levels sometimes depending on which perspective you view the events from. In this case there is the view of the historian, the view of the participants and the view of those who look at not just the outward cause but any underlying cause or causes. So it could get quite complicated.
In a simple sense it was just a victory of one military power or nation against a much weaker nation. The Babylonians were both numerically and militarily superior, which they would have had to be to succeed in taking the well-fortified and positioned city of Jerusalem. Their habit was to take captives and to use them for various purposes in their own land. This befell many of the Israelites who were not killed in the war.
It could also be stated that king Hezekiah foolishly showed the Babylonian envoys the kings treasury and so they 'filed that away' for future plundering when they were able at a later time, after the fall of Assyria, when Babylon had the ascendency..
Another important cause given by those probing into what some would call 'underlying causes' is frequently declared in the relevant scriptures. A summary of these is simply that the Israelites forsook the one true God of their nation and served the idols, which were the 'strange gods' of foreign nations. There are numerous verses which indicate this, a sample of which is contained below:
6The LORD said also unto me in the days of Josiah the king, Hast thou seen that which backsliding Israel hath done? she is gone up upon every high mountain and under every green tree, and there hath played the harlot. 7And I said after she had done all these things, Turn thou unto me. But she returned not. And her treacherous sister Judah saw it. 8And I saw, when for all the causes whereby backsliding Israel committed adultery I had put her away, and given her a bill of divorce; yet her treacherous sister Judah feared not, but went and played the harlot also. 9And it came to pass through the lightness of her whoredom, that she defiled the land, and committed adultery with stones and with stocks. 10And yet for all this her treacherous sister Judah hath not turned unto me with her whole heart, but feignedly, saith the LORD. 3For thus saith the LORD to the men of Judah and Jerusalem, Break up your fallow ground, and sow not among thorns. 4Circumcise yourselves to the LORD, and take away the foreskins of your heart, ye men of Judah and inhabitants of Jerusalem: lest my fury come forth like fire, and burn that none can quench it, because of the evil of your doings. 11For the house of Israel and the house of Judah have dealt very treacherously against me, saith the LORD. 12They have belied the LORD, and said, It is not he; neither shall evil come upon us; neither shall we see sword nor famine: 13And the prophets shall become wind, and the word is not in them: thus shall it be done unto them. 14Wherefore thus saith the LORD God of hosts, Because ye speak this word, behold, I will make my words in thy mouth fire, and this people wood, and it shall devour them. 15Lo, I will bring a nation upon you from far, O house of Israel, saith the LORD: it is a mighty nation, it is an ancient nation, a nation whose language thou knowest not, neither understandest what they say. 16Their quiver is as an open sepulchre, they are all mighty men. 17And they shall eat up thine harvest, and thy bread, which thy sons and thy daughters should eat: they shall eat up thy flocks and thine herds: they shall eat up thy vines and thy fig trees: they shall impoverish thy fenced cities, wherein thou trustedst, with the sword.
oh em gee.