FlySafair, like other airlines around the world, uses a system called "demand based pricing".
What this means is that the first seats on a flight, sell at the very lowest price, usually at a loss to the airline. As the seats start to sell out, they become incrementally more expensive.
What you'll find is that prices rise quickly on popular flights, like those around holiday times. Flights also tend to get pricier the closer it is to their departure time, because usually they'll be closer to sold out at that time.
The best way to get the lowest fare is to book in advance to ensure that you get those first cheap seats. Here's a video that explains this in more detail: