It's a common phrase to say when something is falling from above. The intention is that the person saying it is above and telling all people "below" to "watch out". Paraphrased: Everyone below Juli, be careful! However, the passage is more poetic, and is saying that everyone needs to be careful because Juli behaves with no respect for privacy or space.

In the playpark, there is a piece of equipment called the slide. Kids climb up steps on the one side, and sit to slide down to the bottom on the metal sloped surface. When at the bottom, a kid's movements are not as controlled as as they could be due to the speed of movement. They could bash into someone at the bottom of the person at the bottom does not get out of the way. The phrase is said by someone at the top.of the slide. This might even be used in a metaphorical way