Tail swishing really depends. In general, it is typically a sign or pain or irritation. That's why when you have a horse swishing it's tail that never has before, you'll want to check out their teeth, back, feet, saddle fit, bridle fit, etc to make sure there isn't anything physically causing them pain.
If that's ruled out, then look to the rider. As the spurs were mentioned, sometimes there's things the rider is inadvertantly doing to irritate or annoy the horse.
If the rider is not at fault and everything has been checked out, then they are just maybe a "tail swisher". Some horses just do it. Compare it to someone that bites their lip or sticks their tongue out or makes a strange face when they are concentrating really hard. Tail swishing can be the same thing.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.