If you look up the word 'subordinate' , it generally gets defined as something meaning 'of less importance'.
Now look back at the sentence. Is there really anything subordinate about that subordinate clause? There are two related ideas - me going to football matches and me wearing a scarf. Me wearing a scarf is 'framed' or 'limited' to a certain time or occasion. From an alternative viewpoint, I'm telling you about something I do at football matches. It's a sentence that is both about met at football matches and my scarf-wearing habits. The two parts of the sentence are co-dependent, they affect each other. I can alter the emphasis by switching it round as with 'I wear a scarf, when I go to football matches' but that's not a clincher in terms of which half the sentence I'm drawing attention to. Ultimately, the matter of whether I'm emphasising one side or another of the sentence will depend on the context of the sentence. Am I answering an imaginary question: 'What do you wear at football matches?' Or 'When do you wear your scarf?' Or both?!
So why are we telling primary school children that this 'when clause' (as we used to call them) is 'subordinate'? For no good or useful reason other than that's what a particular strand of grammar has been calling them for hundreds of years. If this kind of grammar was in any way as scientific as it pretends to be, it would junk terms that are inaccurate, misleading and useless. It would explain why and invite people to comment on their decision. Instead, what happens is that the term turns into stone, sits there as if it's a fact, like oxygen, or World War 2, and children have to learn it, spot it, be told that they are 'right' or 'wrong', teachers measured on their ability to teach it, schools judged on children's scores, ministers draw false conclusions from what they call 'data' and comparisons made between countries on PISA tables which are then used to measure 'competitiveness'.