Shaykh Mansour Leghaei has responded to a similar question as follows:
"The time of the caliphate of Imam Ali (as) was one of the hardest periods in the history of early Islam. For the first time in history, three civil wars were imposed on Imam Ali (as). During the periods of the previous Caliphs several innovations had occurred and were practiced for over two decades, among which was ‘Taraweeh’. Imam Ali (as) had to stabilise the country from all the rebels on the one hand, and correct all the innovations occurring on the other. Undoubtedly, fighting the rebels was a paramount priority.
Shaykh al-Kulayni in an authentic narration quoted a long narration in which Imam Ali (as) listed numbers of innovations and how much he tried to stop Muslims from practicing them. As for ‘Taraweeh’ the holy Imam says:
“By Allah! I ruled that people should not offer any prayers in congregation other than obligatory ones. I informed them that offering the recommended prayers in congregation is an innovation. Then some of my soldiers who were fighting for me cried: ‘O Muslims! The tradition of Umar is getting changed as he (i.e. Ali) forbids us to offer the recommended prayers in groups.’ I was therefore concerned that part of my army revolt against me.” (al-Kaafi, vol. 8 p.63)
There is no authentic narration that Imam Ali (as) ever appointed anyone to lead their Taraweeh, nor that he ever endorsed it. He just gave up on them temporarily for a more important matter (i.e. fighting the rebels). Nowhere in the narrations was there a sentiment of the Taraweeh ‘being more or less spiritual’. It was all about ‘following a tradition set by Umar versus the Prophetic tradition’. Similarly, there is no authentic narration that Imam Ali (as) ever asked them to choose a leader to offer Taraweeh congregationally."
See full article here - http://www.askthesheikh.com/did-imam-ali-allow-taraweeh-prayers/
Sadl al-yadayn - leaving the hands open by the side during prayer - is for sure the more authentic practice and was clearly the practice of the Prophet Muhammad (s) and the people of Madina. It was also the practice of Imam ‘Ali (a) and those who followed him, his Shi’a, and also the later Imams such as Ja’far al-Sadiq (a). It was even the practice of the Khawarij who seceded from the side of ‘Ali!
Basically sadl al-yadayn was the only practice for the Islamic prayer until Mu’awiya commanded people to pray with qabd (folding the hands).
I haven’t read a reason for it but my own understanding is that he may have wanted to test the level of religious control he had over his subjects. Abu Hurayra is said to have been in the camp of Mu’awiya during the civil war but he sometimes used to pray behind ‘Ali. It also created a very immediate and visual indicator of which political side a praying man belonged.
As happened with many other matters of this nature in that time period of the Fitna, the hadith fabricators created fake hadith to retrospectively create support from the Prophet (s) for qabd al-yadayn.
As such, the Sunni schools of fiqh accepted both positions of sadl and qabd. It is interesting to note that all four Sunni Imams of fiqh accept prayer as valid with sadl, and that remains the valid lowest common denominator. And of course, as we know, the Maliki school advocates sadl as the correct approach.