Foreign fighters(AQ, Chechens, Uzbeks) started coming into FATA from Afghanistan in 2002. When PA entered FATA and demanded they hand over the foreign fighters, the local tribesmen weren’t having it and the beef started. This is actually how the TTP came into being.
Pashtun tribesmen culture or Pashtunwali doesn’t allow them to hand over someone in their sanctuary even if it is a bharti rapist...
Anyways, they refused, and PA told them Americans are going to be pissed. Local tribesmen said we don’t give a ****.
This carried on for few years till the Americans carried out air strikes and the local tribesmen set their targets on PA because they didn’t get involved in the beef.
Tribesmen thought PA were gonna be on some ride or die shit, but PA didn’t oblige.
That sums it up in simple terms.