Learn Finnish: Valitimme kipeistä hartioista, kun uusi rinkka alkoi hiertää. - We complained about sore shoulders when the new backpack started to chafe.

The verb valittaa (to complain) governs the elative case (-sta/-stä) when you are stating what you are complaining about. Here it is in the plural elative (-ista).

Is there a specific reason to use rinkka instead of the more common word reppu for backpack?

Yes, a rinkka specifically refers to a large, heavy-duty hiking or trekking backpack, usually with a supporting frame. A reppu is just a standard everyday backpack or daypack.

Why is the verb hiertää in its basic dictionary form after alkoi?

The verb alkaa (to start) requires the next verb to be in the first infinitive (the basic dictionary form). If you used a synonym like ruveta or ryhtyä, you would need the third infinitive illative: rupesi hiertämään.

At this level, does the verb hiertää have other meanings besides physical chafing?

Yes, while it literally means to rub, chafe, or cause a blister, it is frequently used metaphorically to mean something that bothers you, annoys you, or causes friction in a relationship or situation.