Here is a timeline of events of leading up to the coup.
The military coup of 12 October 1999 was preceded by tensions in civil-military relations, particularly due to the Kargil fiasco. Before October 1999, media was rife with rumours of a military coup or the dismissal of General Musharraf.
In October 1999, the then COAS General Musharraf went on an official visit to Sri Lanka to meet the Sri Lankan Army Commander. While he was there, prime minister Nawaz Sharif dismissed General Musharraf on October 12, and nominated the then DG Inter-Services Intelligence, Lieutenant General Ziauddin Butt as the army chief.
According to a report, ‘South Asia Plot To Kill Coup Leader’, published by BBC News on 14th October 1999, Nawaz Sharif ordered the plane to be diverted to India. This did not happen and the plane made its way to Jinnah International Airport in Karachi. CAA was reportedly also ordered to turn off the runway lights to prevent the plane from landing.
However, the military police took control of the airport and the control tower, allowing the plane to land. Meanwhile, troops took control of the state-run television and radio stations and encircled the prime minister’s secretariat, and arrested Nawaz Sharif and his brother, Shahbaz Sharif.
At 2:50 am on the 13th of October, General Musharraf addressed the nation. According to a report published in the New York Times, 'Coup In Pakistan: The Overview; Pakistan Army Seizes Power Hours After Prime Minister Dismisses His Military Chief', Musharraf said that the military had overthrown prime minister Nawaz Sharif and his government to stop any further politicization or destabilization of the military.
He added, “Despite all my advice, they (the govt) tried to interfere with the armed forces, the last remaining viable institution in which all of you take so much pride and look up to at all times for stability, unity and integrity of our beloved country.”
Moreover, according to reports published in Dawn on 13th October, the military, in the first few hours of its action on the night of 12th of October had cordoned off Raiwind and Model Town, sealed federal offices including those of civilian intelligence agencies, and had arrested federal and provincial ministers of the Pakistan Muslim League-Nawaz.
Furthermore, bank accounts of many politicians were frozen. The coup reached its completion as the military sealed the office of the prime minister and the leader of the opposition and took over the parliament house.
On 14th October, the constitution was suspended after the declaration of a ‘state of emergency’ and issuance of a Provisional Constitutional Order by General Musharraf.
As of this day, former COAS General Musharraf has still not been held responsible for his actions in October 1999. On the event's 20th anniversary, people are still lamenting the fact that Musharraf has not been held accountable for his actions.
https://twitter.com/ammaralijan/status/1182916682098073601?s=20
Journalist Murtaza Solangi, expressed how Musharraf had resigned in August 2008 instead of facing impeachment.
https://twitter.com/murtazasolangi/status/1163032346498621440?s=20
Twenty years on, there are conflicting arguments as to why the coup occurred. While some argue that the coup was pre-planned and took place due to Nawaz’s interference in the military, others have opined that relations had soured after the Kargil incident and Nawaz’s decision to dismiss Musharraf was the reason for the coup.