Picture the airplane on the ground as a body pivoting about the main wheel axle. In a tailwheel airplane the CG — the airplane's effective mass — sits behind that pivot. Now imagine the airplane begins to swerve, say to the left, during the takeoff roll.
Because the CG is behind the pivot, the airplane's momentum carries that heavy tail outward — to the right of the new direction of travel — which rotates the nose even further left. The yaw feeds itself. Left unchecked, the swerve tightens into a ground loop: a rapid, uncommanded rotation about the vertical axis that can drag a wingtip, collapse a gear leg, or scrape a wingtip on the runway.
Contrast the tricycle airplane: with the CG ahead of the mains, a swerve throws the heavy nose inward, opposing the yaw. The airplane straightens itself out. This is positive directional stability on the ground.
The destabilizing effect of the rearward CG is present any time the airplane is rolling and the tail is light enough to be influenced — that is, throughout taxi, the entire takeoff roll, and the entire landing rollout. It is strongest when:
This is why the most dangerous moments are just after touchdown and during the high-speed portion of the takeoff roll — exactly when the tail is transitioning between flying and being firmly planted.
A tricycle-gear airplane forgives lazy feet. A taildragger does not. The pilot must supply the directional stability that the airframe lacks, using:
The golden rule: stay ahead of the airplane and stop the swerve while it is small. A 5-degree heading excursion is trivial to correct; a 30-degree one may already be a ground loop.