A pledge binds al-Qaeda to Taliban. Why is it a worry for Pakistan?

In 1990s, Osama bin Laden, the founder-head of al-Qaeda, had pledged allegiance called "bayah" to Mullah Mohammad Omar, the Taliban founder-chief. However, there is a deeper worry for Pakistan as al-Qaeda is not the only terror group that has pledged "bayah" to the Taliban.

In the run up to the Taliban takeover of Afghanistan, the United Nations-sanctioned terror outfit had agreed in its peace deal with the United States that they would not allow al-Qaeda or any other terror group to operate from areas under their control. However, only a few days after they took over Kabul, the Taliban gave a clean chit to al-Qaeda. The Taliban said there was no evidence to suggest that al-Qaeda was involved in the 9/11 terror attacks on the US. It was the strike that brought the US to Afghanistan in 2001 with the sole purpose to 'punish' al-Qaeda for challenging the might of the superpower and killing about 3,000 people on American soil.