When does gluten start to develop in pie crust or biscuits?
Wheat and (other grains) contains the two protein classes gliadin and glutenin, which together can form the composite protein gluten.

This process requires water and is influenced by  

a) the amount of water available and
b) the mechanical process of kneading.

Thus a strong gluten network (as desired in bread baking) is acchieved by either 


dilligent kneading wet flour for a certain time (-> window pane test)  
or a higher hydration (= wetter) dough and longer resting times, aided by the -> stretch and fold technique.


On the other side of the spectrum are  flaky and crumbly goods like shortbread , pie crust and bisquits - here you try to avoid excess gluten formation. There are two ways to do this, usually employed both:


Adding fat to the dry ingredients before adding any liquid.
The fat (butter, oil, lard...) "protects" the flour from "getting wet" once liquid is added, thus inhibiting gluten formation.
Using only a limited amount of liquid and no kneading and gentle handling of the dough after the addition. 


So to answer your question:
There is only very little water in your butter which could trigger the development of gluten. Therefore you can "rub" your butter/shortening/... in without much influence on gluten formation. Once you add liquid (egg, ice water, milk, depending in your recipe), you need to be careful.