Why did Pakistan get involved in Soviet-Afghan war from 1979 to 1989?

It's important to remember that the Afghan-Pakistan relations were tense and complicated before the Soviet invasion. The nationalist regime of Daoud initially supported the Baloch insurgency in Pakistan and openly called for the establishment of a Pashtunistan state--i.e. the splitting of Pakistan (probably as a first step towards annexing Pashtunistan). And in turn the semi-autocratic Bhutto government supported cross-border islamists in Afghanistan who were opposed to Daoud. (Some had fled to Pakistan in 1973-75. Gulbuddin Hekmatyar and Ahmad Shah Massoud were both in Pakistan before the Soviet invasion!)

It didn't help that in decades earlier the Soviets were overtly on India's side in the Kashmir dispute, e.g. vetoing UN resolutions that favored Pakistan and that Khrushchev had also been talking up the Pashtunistan project, although he mostly used it to demand Pakistan's "neutrality" in exchange, i.e. that they drop all ties with the US (or else).

In 1976-77, there was a mini-thaw in the Afghan-Pakistan relations, mostly due to US involvement in brokering an agreement that both sides cease sponsoring insurgency in the other. (Iran which was still a US ally at that point was also involved in brokering this deal.) But it was only a partial resolution because other matters like the border dispute were not settled. And soon enough the "Saur Revolution" resulted in Daoud's death, while Bhutto was deposed by Zia. Many on the Pakistani side (Zia in particular) saw Daoud's fall as Soviet involvement.

Zia's regime was even less democratic than Bhutto's. But the Soviet invasion and subsequent US interest in countering it allowed Zia to get the US to look the other way in a number of areas, democracy and human rights on one hand, but also Pakistan acquiring nuclear weapons. (There's a somewhat funny 1981 confrontation between the CIA director, who put the evidence in front of Zia, and the latter's non-denial denial. But more importantly the US didn't really do anything about it.)

The mutual relationship between Zia and the communist Taraki (who briefly replaced Daoud, but before there was an actual Soviet invasion) was also rather abysmal, with Zia being rather religious but Taraki openly mocking that in their first meeting. Zia consequently saw the Taraki government as an even worse enemy than the [somewhat leftist] Bhutto gov't it had deposed domestically and around 1978 Zia made plans to renew the insurgency in Afghanistan, which essentially was underway again before the Soviet tanks rolled over the border. (The latter happened only after Hafizullah Amin killed Taraki and the Soviets intervened to depose Amin.)

Zia's disdain and fears can be read in a 1978 letter to Carter...

Paragraph 8 of the ruling Khalq Party’s manifesto issued in October 1977, specifically mentions the Durand Line, the recognised international frontier between Pakistan and Afghanistan, as a “colonial imposition” and pledges support to the so-called “national movement of the people of Pakhtoonistan” in our territory. [...]

The [Taraki] regime’s lip service to Islamic values and non-alignment is transparent enough. The need for such camouflage to gain time for the regime to consolidate its position, is evident.

If we go along with these professions in an effort to ensure the continuity of the dialogue we had initiated with President Daoud, this does not mean that we are under any illusion or have been lulled into a false sense of security by the regime’s protestations. We know that the Afghan barrier has been breached and our country lies directly in the path of the flood which rolled out of Czarist Russia in the last century and is now flowing in full force towards the Indian Ocean and the Persian Gulf.