A few factors to consider. First, antiquity and late antiquity were littered with examples of military leaders translating success on the battlefield into political power. Think of the late Roman soldier emperors, e.g. Diocletian. Whether or not you believe that spontaneous acclimation by a commander’s army was genuine or not, it had happened often enough before to be a serious threat.

Even if Belisarius wasn’t aiming to usurp Justinian’s position, it was equally plausible to believe that he was attempting to set up an independent power base. His ruse to end war with the Goths by assuming the title of King of Italy was more than plausible for both sides. The fact that it wasn’t real still made it a potential threat and could have been seen as a trial balloon later abandoned. The Byzantines struggled to project power at that distance, so it would have been difficult to dislodge Belisarius if he had actually chosen to go rogue.

Second, Justinian’s position rested on a relatively thin base. His uncle Justin’s assumption of the purple had been effected in no small part through bribery after the death of Anastasius. There was no direct connection to the previous emperor through blood or marriage. Additionally, Justinian’s marriage to Theodora was viewed with suspicion or outright disgust by many members of the Byzantine upper class, which further weakened his position, at least until sometime after the Nika Revolt.

While no one factor may have been decisive in Justinian’s relationship with Belisarius, they all would have been somewhere in his mind as the general regained territory after territory, amassing power and prestige.

Justinian had sent terms to the Goths, which the Goths agreed, that everything south of the Po River was Byzantium. Belisarius in fact called a counsel of war in which all commanders present (besides Belisarius I guess) agreed - in writing because Belisarius asked them to set it down in writing - that going with the terms Justinian set forth was the best course of action.

Only after that did Belisarius tell the Goths they should make him emperor so he could capture Ravenna.

Belisarius there-by directly disobeyed if not Justinian's direct command at least his wishes. Prior to carrying out the deceit, Belisarius also sent away all the other major Byzantine contingents in the vicinity of Ravenna, except his own. Belisarius' personal wishes aside, I hope you'd agree that would seem very suspicious.

But even then Justinian didn't sabotage Belisarius' campaign. Procopius outright states that Belisarius was originally recalled not because of Justinian believed rumours or slanders, but because Belisarius was needed for the war with the Persians. When Belisarius was sent to retake command in Italy, he had to leave the army on the Persian front and raise a new one. For the rest of the campaign, unlike the successful one before, Byzantium was fighting a two front war. Not only that, the Huns had at the same time attacked Illyria (which, and not being paid, caused the Illyrians under Belisarius to go home without orders). It could simply be that Justinian had no more troops to spare for Belisarius...except for what he did send.

For Justinian did sent reinforcements to Belisarius. When Belisarius waited in Ravenna and sent one of his commanders to ask Justinian for troops, none were given. However, later when Belisarius decided to go himself to Epidamnus (in modern Albania) to ask for reinforcements, Justinian gave him a new army. One which Belisarius was able to divide up into two or three forces for independent operation, so it must have been sizeable. And later when Belisarius was busy fighting the Goths around Rome, Justinian again, on Belisarius' urging, sent him more reinforcements, which landed in southern Italy. With this, according to Procopius, Justinian wrote to Belisarius that "he had sent him a numerous army with which he should unite in Calabria and so engage with the enemy." So Justinian thought the forces he sent Belisarius was adequate, and there's not really any evidence to suggest otherwise. And then yet again later on, when Belisarius was momentarily forced to retreat to Sicily following a defeat, Justinian sent him more reinforcements.

With that in mind, Justinian in fact seemed to have supported Belisarius whenever he could. Even Belisarius' second and final recall was because of looming war with Persia. That the Byzantine commanders in Italy couldn't properly co-ordinate with each other and the fortunes of war was with the skilled Gothic king could certainly not be charges against Justinian.

Thanks all!