Structure is the proliferation of agency. This is what Latour is saying, I think; maybe he even said it that directly - I can't remember. In other words, we create structures by coming into relation with others around us - human and non - and in doing so we create new agencies (saying agents sounds wrong here...too discrete, encapsulated, or isolated). It's these agencies that we refer to as structure - they constrain and enable our ability to act in the world.
It only makes sense... the more other agencies there are - the more entities we must interact with - the more we'll feel moved in certain directions. It helps also when these agencies tend to line up in certain directions, but that's not always the case - is, in fact, unlikely, I suspect. Why didn't we see it sooner? We already talk about structures as if they have agency - constraining and enabling are both active engagements. The problem is that we abstracted Structure and failed to see the others agencies around us. We wanted to see Structure as one big thing, and not a proliferation of myriad little things (agencies).
Ooo... here's a thought... agents of structure... like agents of the CIA, the Kremlin or the Gestapo. They're undercover - we don't know they're there! Best not to think about it too much... that way lies paranoia!