EXTREME ZOOM-IN. Looks like the other girl was peeking over the pier's edge while Saki was looking over her shoulder. My guess is that blondie
did fall over, but like in Ano Hana, Saki wasn't able to do anything about it.
Also, the blow-up confirms that the girl is likely a Miyanaga relative. Hat aside, she and Saki share the distinctive sharp point in their hair, which has never been seen on any other character aside from Teru.