To be fair to Leese, Ronald Lewin puts this down to c*ck up rather than any deep conspiracy; Robert Lyman is less kind but suggests that Leese felt he needed someone with whom he had a better personal relationship. Either way, Leese's basic premise was that Slim needed a rest and that his lack of expertise in amphibious ops would put him at a disadvantage for Op Zipper, suggesting a change in command arrangements.
Leese proposed to Mountbatten that when XIIth Army formed, with the task of mopping up in Burma, XIVth Army went on to conduct Zipper. Slim would move to XIIth Army. XIVth Army would go to Philip Christison.
Mountbatten thought otherwise, and suggested that resting Slim and making a temporary appointment to XIVth Army would be a better bet.
Leese was to approach Slim, sound him out about having a rest and moving to XIIth Army, but on no account was he to do anything that was contrary to Slim's wishes. If Slim wanted to stay with XIVth Army, then that - as far as Mountbatten was concerned - was that.
Sadly, Leese told Christison (who was a friend) he was to take XIVth Army *before* he spoke to Slim and gave Christison the impression that Slim was going on indefinite leave and not taking any future command. One of Leese's most tactless staff officers went to see Slim and gave Slim the impression he was being sacked; Slim insisted that if the C-in-C Land forces in SEA was getting rid of him, he should at least have the decency to do so himself.
Leese then saw Slim and Lewin suggests he then compounded the comms c*ck up by confirming Slim's impression that he was to hand over command of XIVth Army and take on XIIth Army after some leave, rather than simply sounding him out about the idea which was all Mountbatten would let Leese to do - his plan, I suspect being that Slim would say 'er, not happy about that idea, sir', whereupon Leese would say 'He doesn't like the idea, sir', to which Mountbatten would say 'told you so; give him some proper leave and tell him to be back on such-and-such a date'
Slim thus thought that he'd been sacked from XIVth Army, and told Leese that he thought that as it seemed he'd lost the confidence of commander land forces, it would be better if he didn't take XIIth Army and instead retired. Slim then told his wife and staff he'd been fired, while Leese only slowly began to appreciate that he'd handled it badly, while the XIVth Army staff were aghast to the point that Slim apparently had to dissuade several of them from resigning (Lewin suggest that one of the senior RAF officers also had to be persuaded not to resign).
Full appreciation seems to have arrived only when Brooke sent a missive best summarised as 'What the Fcuk is going on, Dickie?' to Mountbatten.
Once that arrived, Leese was effectively done for. Slim was told he was to go on holiday - which was what Mountbatten had wanted Leese to sort out for him in the first place - while Christison would take temporary command of XIVth Army, and Stopford would take XIIth Army. Slim's holiday (taken back in Britain) concluded with him being informed that Leese had been sacked and he was to replace him.
Three answers therefore suggest themselves. First, that Leese was desperate to replace Slim and sought to ease him out with the offer of a good rest and then a new command, in the hope that he'd agree to go. Mountbatten would be faced with a situation where Slim had agreed to Leese's general premise, leaving Mountbatten without room to manoeuvre.
Second, that Leese simply made a complete horlicks of carrying out Mountbatten's instructions and gave Slim completely the wrong impression, again with disastrous effect on Leese's career.
Third, that Leese deliberately disobeyed orders, told Slim he was to take XIIth Army with the notion that Slim would obey this instruction without demur, whereupon Leese would tell Mountbatten that after sounding him out, Slim had agreed to go. Mountbatten then believes Slim has agreed to go, rather than having obeyed orders to accept being pushed sideways.
Personally, I suspect that you have a mix of one and two above - Leese's over-eagerness to move Slim foundering when his skills at persuasion failed disastrously, creating a situation that he hadn't quite intended and didn't know how to deal with.