They did the exact same thing for TREME and for BOARDWALK EMPIRE: built up the first episode hugely, marketed the hell out of them, got people to watch, renewed immediately, and then they had two seasons to address any ratings fluctuations (both TREME and especially BWE dipped massively after their first episodes) before having to decide on a third season.

I think it was inspired by TRUE BLOOD, which got poor ratings when it started, enough to worry HBO, but they kept at it (helped by the fact it's relatively cheap) and built it up and got the first season DVDs out and then the ratings improved rapidly in Season 2 and it became a break-out hit.

In a way it's a bit corny. We knew they were going to do it, so some have argued that HBO giving the green light to a second season some time ago, or even greenlighting the first two seasons at once (something that was briefly rumoured for THRONES) could have been seen as a huge display of confidence for the show and would have been more impressive. But I guess HBO wanted to retain flexibility just in case it bombed massively out of the gate.