First what is really happening? Well I encountered this mid-sized greeny in shallow water (3m or so) while snorkelling. She was pretty chilled while I swam alongside or above him but here she's just taken a breath and has hit the gas immediately thereafter.
Why the tiger shark perspective? Well tigers (worldwide - we don't tend to see them here very often at all unless there is a major food source in one place) tend to like to hunt shallow sand flats and sea grass beds and often they hunt air breathing species like turtles, dugong, dolphins and so on. Whilst these species are in midwater they are safe as they can probably outspeed or outmaneuver or otherwise escape. Feeding on the bottom they are a little exposed as the shark may be able to surprise and kill them by bleed out or drowning. Breathing on the surface they are really exposed as they have a huge blind spot and nowhere to go to.
That's why I call this the tiger shark perspective, why this greeny decided to book it and why if you want to get close to sea turtles you don't approach them quickly from above or below but should move in slowly on a level.
Here's another greeny from the same day (slightly younger) considering its options in light of the big animal following it. This one has been tagged by our conservation dept (front right flipper - not visible in the picture) and has therefore probably developed a healthy dislike for humans approaching from anywhere never mind sharks.