I think people are taking this to the extreme and talking about what would happen if the queue piles up behind her. But on a less dramatic level, maybe she's just finishing the page of her book she's reading, or finishing off writing a long and potentially important text - tasks which you'd rather get done in one go. The gap between her and the next person in queue is small enough that it could've happened all in one queue movement, and she's just not instantly moving - she wants to finish her page.
Having been in queues in airports, the pressure put on you by the people behind you is ridiculous. It feels like if you spend 30 seconds not moving (nowhere near enough time to cause a pileup that spills into the walkable areas) people think you're crazy, when in reality it just means picking up your bags less and getting to finish reading that task you're doing