The article describes the microwave link as 'additional capacity', so I'm guessing that they had another (primary?) connection, but needed more throughput.

When the rugby world cup was running last year I saw a number of temporary cell sites, which had plastic ducting and fibre optics running ~50-200m to existing telecoms cabinets or cell sites.

I imagine that At&t is doing something similar, hooking into an existing cable at the nearest roadside cabinet or traditional cell site.