It seems that Liu Shaoqi fell out of Mao grace very quickly and was treated worse than other leaders that just criticized Mao. Was there more to it? Was it becouse of power held by him?
To answer this question, I am going to rely on a translation corpus of first hand accounts of the Cultural Revolution from the Lushan Conference (1959) to 1970 compiled by Belgian author Simon Leys in 1970 (see at the end). Please keep in mind that his work is most and foremost journalistic in essence, given the very little hindsight the time between the start of the R.C. (1967) and his work (1970) allows for. Many, however, praise Leys' book as one of the best contemporary resources on the R.C. in the French-speaking world.
The starting point of my answer has to be Peng Dehuai's condemnation of Mao's Great Leap Forward, which to Peng had been a disaster on all levels. Peng Dehuai had been a close ally of Mao's, especially during the Long March. While he initially expressed his doubts to Mao in a private manner, the latter saw Dehuai's perplexity as an affront and invited Peng to disclose his ideas in the scope of the 8th Central Committee of the Communist Party of China, also known as the Lushan Conference (summer 1959). Virtually all participants argued strongly in Mao's favor; Liu Shaoqi was one of them.
As it is the case for many relationships, the Mao-Liu relationship was rather complex. In all regards, Liu Shaoqi was considered by many to be Mao's designated successor and one of his closest allies prior to and during the Lushan conference. The latter was seen as an earnest follower of the former, and his nomination after Lushan as the de facto leader of the C.C.P. somewhat alleviated Mao's ire and (perceived) humiliation Lushan had caused. Indeed, Liu endeavored to follow a strict Maoist line while in office.
However, because he lacked "personal ambition" (p.43), Liu was overwhelmed by how complex the Great Leap Forward's legacy was to handle. Practically speaking, Liu Shaoqi had two main issues he had to solve: first, the Great Leap Forward's momentum grew more problematic by the day and had to be halted, one way or another. Second, now that Mao wasn't the C.C.P. number one figure anymore, voices were rising in favor of Peng, arguing that (1) his treatment was unfair, and that (2) he'd actually made solid points at Lushan. After all, the G.L.F.'s flaws and consequences were evident to many (p.44). Thus, Liu Shaoqi proceeded to formally condemn Peng Dehuai but did not convict the latter. Furthermore, his policies following his investiture were de facto applying parts of Dehuai's observations as the C.C.P. proceeded to put an end to the G.L.F. while at the same time mending its disastrous consequences. Between 1959 and 1961, People's Communes were terminated, and so called "exaggerated" crop counts (one of Dehuai's initial observations), were addressed. Leys goes to explain that, by 1961, the Great Leap Forward was a relic (p.45).
In the following months, Liu understood how crucial it was for an event such as the G.L.F. not to happen ever again and strove to consolidate the new post-G.L.F. status quo by strengthening his cabinet's affinity to his practicality all the while relying on intellectuals to corroborate his anti-G.L.F. policies. Intellectuals such as Wu Han and Deng Tuo would go on to publish articles, short stories and even plays which defended both Liu and Peng Dehuai's stance, all the while criticizing, albeit in a most subtle manner, the Great Leap, Mao's term in office and Mao himself. Intellectuals acted as fuses for the C.C.P. who wanted to maintain and consolidate China's transition out of the Great Leap Forward without directly opposing Mao, who still enjoyed a great deal of ideological influence. Pro-Peng and anti-Mao literature (think Hai Rui's Destitution by Wu Han) would prove instrumental in Mao's return to power in 1967. In 1961-1962, the C.C.P. would go as far as to overtly criticize the Great Leap Forward in several official reports. All these policies also aimed at weakening Mao's aura and legacy, as the perspective of him coming back to power was ominous to many.
Liu's cabinet (with the notable exception of Lin Biao) was more than fine with Mao as a Chinese Lenin, half alive, half dead, cautiously displayed as an almost divine relic of the P.R.C.'s history. However, Mao was more of a prophet than he was a god ; people listened to Mao. His various campaigns aiming at impassioning the military and the youth into fending off "revisionist" and "capitalist" threats (both among the Chinese people, the C.C.P. and especially intellectuals) would eventually prove successful to the dismay of Liu and most of his cabinet. As the R.C. wasn't part of your initial question, I won't be covering it.
Going back to what the Liu cabinet did and implemented, Mao Zedong was painfully aware of it all. He witnessed Liu's gradual estrangement from a strictly Maoist stance to a revisionist point of a view that, in addition to being blasphemy in regards to Mao's work, was a personal attack. Liu Shaoqi would go on to be one of the R.C.'s prime target, before dying in 1969.