Yes, the SA code.

No, increased demand because of Machu Pichu trekkers does not account for much, there are not THAT many, (and even then, if the plane is full of demand by trekkers then why offer a discount?). Year round this route has many many Spanish speakers. Most demand is Christmas and January (the nation summer vacation is the month of January in Peru), biggest travel period nationally and internationally for them.

I should note this route escaped the recent AE rewards enhancement so that says a lot about demand...

But back to topic. They increase the fares then they promote a discount worth half what they increased the fares by, and after the validity period of the discount, the prices go back down to normal. Marketing trickery IMHO.

AND the discount code does not work on any low fare you can mistakenly find, only on the higher fares.