The debate between Byzantine iconoclasts, who believed that a cult of icons was immoral, and iconodules, who believed it was the right way, lasted for over a century and a half, but in two main periods: 726-787 and 814-843. I can therefore only answer why iconoclasm first came about in the early eighth century, anymore and it would be an essay.

Although the answer extends beyond a simple theological discussion, the Second of the Ten Commandments in the Bible does indeed ban worship of "graven images." This was backed by early Christian theologian Eusebius who wrote letters criticising the adoration of icons. Despite this, Byzantium hardly iconoclast prior to 726. Theophanes records Emperor Heraclius (610-641) putting icons of the Virgin Mary on the prows of his ships upon sailing to war and Theodore the Synkellos tells us that the Virgin Mary was painted on the gates of Constantinople to evoke her protection of the city. In 692 the Quinsext Council decreed that a human version of Jesus should always be depicted, rather than a simple metaphor for him. Soon after, images of Christ could be found everywhere in Byzantium, on the walls of the imperial palace, even on the coins.

So what changed in the next few decades? The personality of Emperor Leo III (717-741) must be pointed to as a factor. Prior to his coming to the throne, Leo had been a strategos of a theme (military-controlled province) that lied on the border with Islamic territories. It is more than likely that a cultural and religious osmosis occurred that brought Leo into contact with the iconoclastic teachings of Islam. Theophanes himself says Leo was "Saracen minded." To add to this, an associate of Leo's, Beser, was possibly a convert to Islam and was known to have the emperor's ear. It is entirely possible that Leo listened to the ideas of Islam, and it may have also struck him how successful Islam had been in taking Byzantine territory in the previous century, when the Empire was not iconoclast.

Another pseudo-religious motive is mentioned in the Letter of the Three Patriarchs. The letter talks of Jewish magicians coming to see Leo III when he was still a peasant, before he became strategos. They told him that he would become emperor, but to do so he must destroy the holy icons. The chances are that this story is largely apocryphal - meant to reveal that Leo was a bad emperor for consorting with Jews.

A more secular reason for iconoclasm was because of the threat of holy men. Holy men and monks had something called "parrhesia," which basically meant that they had a direct relationship and conversation with God. This made them virtually untouchable, giving them freedom to criticise whomever they wanted. Holy men were serious iconophiles and monasteries were centres of iconophilism, so to deem iconophilism heretical would take away their protection.

Despite these reasons, it may have been simpler than that. Leo III had been a lowly peasant, then strategos, only becoming emperor after staging a coup. He may have simply desired to mark his meteoric rise with a groundbreaking legal and doctrinal upheaval.

My apologies for not extending this answer to further years. If there is interest then I can expand more into the extent of change that iconoclasm brought.

Source - Peter Brown, 'A Dark Age Crisis: aspects of the Iconoclastic controversy,' English Historical Review, 1973. Link.

Since your question invokes that ever so complicated word "real", I'm going to pass onto you an alternate explanation for Byzantine iconoclasm, as suggested by historian Mark Whittow.

The 7th century Arab Conquests were dramatic as well as traumatic. Coming after the recent victories of Heraclius against the Sassanid Empire, which was seen as a triumph of Christianity (especially considering how close the Roman Empire came to extinction in that fight), to go from fantastic victory to defeat after defeat by the Arabs would suggest to the mind of the Byzantines no cause other than anger from God. But what was the reason for anger? The most obvious thing they could think of was changes to the theology, of which Heraclius' recent attempts to end the monophysite schism with a compromise doctrine of monothelitism come to the forefront.

Don't think this religious reason is too strange, as even in our day and age, religious leaders are constantly citing moving away from God as the reason for our disasters. Hell, consider too how often we simply blame "the party in power" (as a replacement for belief in what god believes) for downturns in the economy, and simply vote them out, regardless of whatever actual effect they had on the economy. It's an equation that makes logical, if not evidentiary (which is a modern anachronism anyway) sense.

As the losses piled up, a search was on for changes to alternate theologies in order to appease what was undoubtedly a wayward movement against "orthodox" thought. What came around was the doctrine of iconoclasm, at the time of the rise of Emperor Leo III, who began to turn the tide of the Islamic advance. With military victory came religious victory, and a vindication that iconoclasm was god's chosen philosophy.

Now one thing Whittow always wants us to keep in mind, is that if you take a look at the main changes back and forth between iconoclasm and "orthodoxy", they actually line up quite well to the political changes in Constantinople, as well as the military changes in the neighboring Caliphate.

Meaning as the Caliphate experienced civil war and weakened, Byzantine forces expanded in that presence, and expansion coincided with a change in theological philosophy. Leo III comes aboard with iconoclasm to turn the tide of the Arab conquests, this happens exactly when the Umayyad Caliphate is collapsing into civil war. After a period of stability, iconoclasm is done away with when Empress Irene came to power. She came to power in a coup, which required she craft a bureaucracy that was loyal to her and not to her predecessors. The best way to do this would be by surrounding herself with iconodule supporters. When Empress Irene was overthrown, iconoclasm came back. However when the emperors after her failed to secure victory against the arabs and suffered a series of defeats because they were now up against the rising Abbasid Caliphate, and thus the population sees this as a sign to turn away from iconoclasm.

Which is what gives us an alternate, or "real" if you so like explanation for iconoclasm. Political forces inside Constantinople as a reaction to political changes outside the empire. The theology itself is merely the "legal" justification for the changes, but the practical ones are dictated by the military and the political situation.

There's also an economic and regional twinge to the iconoclasm debate too that I don't know quite enough to get into, but this is just to indicate that religious philosophy was not the only mover in the controversy.

Source:

Whittow, Mark. The Making of Orthodox Byzantium, 600-1025. Basinstoke: Macmillan, 1996.