ChiSau,
It's a tail-dragger thing.
Imagine holding a pencil vertically. If you hold it from the top, gravity will cause it to hang straight down, even if you don't grip it very tightly. This is called "stable". Try holding it loosely from the bottom, though, and you'll have to juggle with it a bit to get it to stay upright. This is "unstable".
The Cessnas/PA28s that you learn in are basically stable aircraft on the runway. If the nose is pointing slightly to the left, the momentum of the aircraft (which acts through the centre of gravity, i.e. in front of the main wheels) will pull the aircraft straight. Imagine a piece of string tied to the centre of gravity of an aircraft, then pull that piece of string not quite straight forwards, but a few degrees off to the side - the aircraft will soon line up with the direction you are pulling the string.
Now imagine the same piece of string on a tail-dragger, which is unstable. If you pull the string any more than a few degrees to the side of where the aircraft's nose is pointing, it won't straighten up at all - the back of the aircraft will spin around until it's pointing the other way.
This is a ground-loop. At best, it's embarassing. At worst, the aircraft could tip over, damaging a wing-tip or worse. The way to avoid it is to ensure that every time you land, you are pointing straight down the runway, with no drift at all - things you should do in a tricycle-geared aircraft, too, but they're especially important in a tail-dragger for this reason. Also, when taxying, be ready to correct any deviation from where you want the aircraft to be - a slight gust of wind can catch the tail and induce a ground-loop at very low speeds.
Does that answer the question?
FFF
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