Basically, many informed observers seem to think that Dr Mahathir decided Anwar could no longer be trusted to protect Dr Mahathir's interests anymore, especially after Dr Mahathir is no longer PM. the transcript

Dr Mahathir was certainly not too pleased with various things Anwar did from mid-1997. When Dr Mahathir went away for two months, Anwar gave the impression that he was going to be tougher on corruption. Then after Dr Mahathir took over economic policy after his return, the foreign media began mocking his conspiratorial analysis, generally running him down and promoting the idea of an early Anwar succession. From the end of the year, Anwar seemed to take over economic policy, cutting government spending, raising interest rates and tightening liquidity, which arguably exacerbated the crisis and took the economy into recession in 1998, especially after the Kongsi Raya holiday reprieve.

But I think the straw which broke the camel's back came around late May or in June, with developments in Indonesia and the subsequent adoption of the reformasi slogan and the anti-KKN (corruption, cronyism, nepotism) campaign by the UMNO Youth leadership then, who were close to Anwar. I don't think Dr Mahathir minded attacking korupsi and kronisme, but nepotisme came too close to the bone. Several months earlier, PRM president Dr Syed Husin Ali and a couple of associates had asked the Anti-Corruption Agency (ACA) to investigate how Dr Mahathir's three sons had gained stock in over two hundred companies by late 1994. Soeharto's resignation on 21 May and the continued attacks on the ex-president who had only recently joined the ranks of Forbes magazine's richest men in the world - after the Sultan of Brunei and Bill Gates – must have upset Dr Mahathir even though there are important differences between the two.

I am not sure; many believe that some of Anwar's enemies had hatched up the ‘plot' to finish off Anwar politically before that, but Dr Mahathir still felt Anwar was the Prime Minister's least problematic option then, and was not yet willing to go along with them at that point.

There were Anwar's critical Johor speech, the unevenly attended Pemuda economic convention a couple of weeks before the late June general assembly and Zahid's speech at the UMNO Youth assembly itself. Anwar's assembly speech did not criticise Mahathir at all, and in fact announced a U-turn from his December 1997 economic policy, by increasing government spending and liquidity and trying to lower interest rates, almost as if in response to Daim's and Dr Mahathir's earlier criticisms.
Others Anwar had consulted had voiced similar concerns as well. Maybe he was keeping his cards very close to his chest, but Anwar did not respond positively, for example, to those who called for him to ‘lead us out of this darkness' and even went out of his way to explain Mahathir's concerns.
There is little evidence of any serious effort by Anwar's camp to mobilise forces and resources to actually try to oust Dr Mahathir. Pointed criticism of nepotism, yes, but a effective plan or strategy to oust Mahathir, unlikely. And if there was one, it was terribly amateurish and bound to fail. But whatever it was, it was enough to convince Dr Mahathir that Anwar was out to replace him.

Dr Mahathir was very cool. I saw him smiling proudly at the St Petersburg Orchestra's concert at the Petronas Philharmonic Concert Hall the night before he delivered his devastating rounding-up speech and released the partial lists of tender, contract and privatisation beneficiaries from the Economic Planning Unit (EPU) and the Ministry of International Trade and Industry (MITI), all of which reflected sound preparation.

Although it was later evident that a purge of Anwar's camp had begun, beginning with the media, I thought that Dr Mahathir had Anwar exactly where Dr Mahathir wanted Anwar - weakened, compliant and constrained from mounting an effective challenge. I wrongly thought Dr Mahathir would prefer the safety of such an arrangement rather than risk an Anwar challenge by sacking him or forcing him to resign.