is there any other specific reason for it?

The reasons for the invasion of Sicily by a conglomerate of Islamic factions are... weird. You probably didn't think the answer to this question would involve nuns and noses, but it does.

In 826 the commander of the Byzantine naval forces stationed in Sicily was a man named Euphemius. He had been promoted early in the year and energetically developed Byzantium's naval strength in the area by attacking targets along the African coast. However, his success went to his head and led him to believe that he could do pretty much whatever he wanted. According to the continuation of Theophanes Continuatus' chronicle, that included forcing a nun named Homoniza, whom he fancied, to marry him. He did this by abducting and then threatening her. The Chronicon Salernitanum tells a slightly different story, in which Homoniza was betrothed to Euphemius by the governor of Sicily and then married off to someone else at the last minute, leading Euphemius to take revenge and try getting her back. Either way, emperor Michael II and his civil servants in Constantinople suddenly had a lot of letters on his desk asking what the hell was going on in Sicily with Homoniza and Euphemius.

Whatever was truly going on, Michael II ordered that Euphemius should be removed from his post and, as punishment, have his nose cut off. It's pretty doubtful that this was all about a nun, it seems likely that Euphemius' attacks on the African coast had not been sanctioned by Constantinople and that Michael feared Euphemius' energy and independent spirit might lead to rebellion. The hope was to nip the problem in the bud and have Euphemius dealt with quickly, but Euphemius somehow learned of his imminent disfigurement and seized Syracuse with his fleet. He was also proclaimed emperor, and began referring to himself as 'Emperor of the Romans'.

But Euphemius had few allies in the army. A few cavalry officers are said to have joined his side but the overwhelming majority were willing to obey the governor and put down Euphemius' attempted usurpation. With an army raised, Euphemius was driven out of Syracuse. He took his fleet to the Aghlabid emir Ziyadat Allah, who ruled a powerful region centred around what is now Tunisia. A desperate Euphemius offered him rule of Sicily in exchange for military support. Ziyadat's army was restless and rebellious, so this was a good opportunity simply to keep them busy whilst bringing prestige upon the Aghlabids. The deal they struck was mutually beneficial, and generous to Euphemius. Euphemius could keep Sicily, and would simply pay tax to Tunisia rather than Byzantium. Their agreement also stipulated that Euphemius open up the ports to Muslim sailors. This meant that Muslim sailors, who often faced hostility around the Italian peninsular, no longer had to hug the African coast. This also made it easier to find shelter during rough seas.

Ziyadat Allah promised that Euphemius could keep Sicily in exchange for a yearly tribute, effectively making it a satellite state rather than a full conquest. His army counted 10,000 infantry, 700 cavalry and 100 ships, reinforced by Euphemius' remaining forces. There were immediate problems. The commander of Ziyadat's army, a man named Asad ibn al-Furat, did not get on with Euphemius. Asad was a qadi, a legal scholar and theologian, who was perhaps not the best person to put in charge of an army. Furthermore, the Muslim forces found it difficult to distinguish between the soldiers of Euphemius and the soldiers of the Byzantine Empire, which resulted in accidental skirmishes. Euphemius very quickly realised that he would be the junior parter in this arrangement, and that Asad had goals of his own.

Asad romanticised the Islamic Conquest of the past, and saw the invasion as his chance to imitate that age. When Euphemius had come before Ziyadat's court the consensus was that they shouldn't help out, but Asad had persuaded them. Once on Sicily with his army, Asad proclaimed that he would continue the invasion without Euphemius, and so Euphemius sent letters to the Byzantine Empire to warn them of what his enormous blunder was about to bring upon Sicily. Asad besieged Syracuse in 827, which dragged on into 828, at which point disease spread through the region and killed Asad. Euphemius was overjoyed, and hoped that the Muslim forces, without their stubborn jihadist at the reigns, would go back to the original plan. To his great relief, the new commander of the Muslim force (Muhammad ibn Abu'l-Jawari) had no problems at all with Euphemius, wasn't interested in chasing past glories, and the deal was back on. They attempted to go back to Africa to regroup, but the arrival of a large Byzantine naval force trapped them on the island. Ironically, it's this victory that put the Byzantines at a huge disadvantage - the Muslims and Euphemius were preparing to go home, when the Byzantines could have used their naval superiority to keep them there. Instead, this ensured that a large and hostile force was trapped, with no choice but to put up a fight. The Muslim/Euphemius force split in two, with Euphemius leading an army to the town of Enna, and Muhammad ibn Abu'l-Jawari marching on the city of Agrigento.

The people of Enna said that they were willing to negotiate with Euphemius, but only if he came in person. Euphemius was naive enough to do this, and was promptly stabbed to death by the emissaries of the town. This left the invaders without their casus belli, but since they were already trapped on the island they either had to surrender or keep fighting, so they kept fighting. They even began striking coins with Ziyadat Allah's name on them! But the Muslim force was clearly on the back foot, and was holed up in the castle town of Mineo with no realistic prospect of victory as the Byzantines closed in. By this point Muhammad ibn Abu'l-Jawari had also died, and left an officer named Zubayr ibn Gawth as the ranking commander.

Then, in one of the many bizarre events of this story, a raiding fleet from Al-Andalus (Muslim Spain) showed up. They had no intention of taking the island, they were just there to steal some Byzantine gold, but the besieged folk of Mineo managed to smuggle them a message and open up a line of communication. Now it was Zubayr's turn to offer up rule of Sicily in exchange for military assistance. The Andalusian fleet's commander was willing to help as long as he was recognised as the leader of the combined force. The Andalusians also informed Ziyadat Allah of the situation, who promptly sent reinforcements and formed a coalition. In 830 they beat back the Byzantines, but their leadership was wiped out by another outbreak of disease, whilst the Byzantine leaders were killed in a skirmish. So by the end of 830, none of the leaders were even alive, there were two armies (one mostly Muslim, one Byzantine) tearing the island apart trying to kill each other, and both sides were piling troops into Sicily hoping for an advantage. In the chaos, a force from Africa managed to take Palermo and establish a secure area on Sicily's western coast. They also formed an alliance with Naples, which had its own problems with the Byzantines. The Byzantine Empire was also being attacked from the east, so it called in regional allies to make up the numbers and keep the fight going whilst forces in Sicily were redeployed to Anatolia.

So what happened over the following decades, which I don't have the space to go through in full, is that each side suffered from horrendous attrition and leadership mortality, which led to frequent changes in the political sympathies of the leadership as different coalition partners held more influence at different times. Also, whenever one side realised that it was losing, it would attempt to drag in more allies into the war. By the 830s, so many factions had a stake in the conflict that it couldn't end until someone won, and even if they wanted to pull out they couldn't because so many allies were depending on them, not least the fledgling Emirate of Sicily. It could be said that the final conquest of Sicily by the Muslim coalition took place because of the sunk cost fallacy - it's not like decades of smashing the island to bits was going to be worth it at the end, but they had expended tens of thousands of soldiers and sailors and that couldn't be for nothing.

The conquest was completed in 1042, just 19 years before the Normans would show up and conquer it for themselves.

So in summary the reasons for the Muslim conquest go something like this:

Helping Euphemius keep his nose -> chasing jihadist glory -> Helping Euphemius again -> We're stuck here -> Help the people stuck there -> Sunk cost fallacy