Learn Portuguese (Portugal): A moda de ganga está a tornar-se mais confortável. - Denim fashion is becoming more comfortable.

In European Portuguese, when you have a continuous verbal complex using estar a plus an infinitive, the pronoun naturally attaches to the end of the main verb with a hyphen. Since this is an affirmative sentence without any 'magnetic' words (like não or que) to pull the pronoun earlier, -se stays attached to tornar.

Could the pronoun attach to the auxiliary verb está instead?

Yes, it is grammatically possible to attach it to the auxiliary verb, giving us está-se a tornar. However, attaching it to the main infinitive at the end (está a tornar-se) is the standard and most natural choice for affirmative sentences in Portugal.

Why do we use the reflexive -se with the verb tornar here?

By itself, tornar usually means to return or to transform something. When you want to express the idea of 'becoming' (undergoing a change in state or quality), you must use the reflexive form tornar-se.

What is ganga and why is there a de before it?

Ganga is the European Portuguese word for denim. English often stacks nouns together to act as adjectives (like 'denim fashion'), but Portuguese uses the preposition de (of) to link them, literally translating to 'fashion of denim'.