Some factors to consider:
A tricycle type is directionally stable ie tends to keep moving in the same direction because the CG is ahead of the centre point of the drag from the wheels.
This means that if the two get out of alignment the inertia of the CG (pulling forward) is acting opposite to the drag of the wheels (pulling backward). Within certain limitations, the combination act as a couple, pulling the a/c back into alignment with its direction of travel.
A tailwheel a/c has the reverse. The drag is centred ahead of the CG. The CG has a tendency to keep moving in the direction it was traveling, while the wheels are trying to do the opposite.
If the wheels & CG get out of aligment with the direction of travel then the a/c will have a tendency to continue to yaw - unless a correcting force is applied. Think of a car skidding.
The correcting forces or factors that are available include keel area & rudder input, tailwheel effects eg locked or not, or steerable, moment arm over which these forces can act, width between the main wheels (Is this called groundtrack? I forget), rolling resistance caused by the surface etc
Spacing of the mainwheels is a factor because the further the wheels are apart the more yaw must happen before the CG moves far enough to the side for sufficient misalignment to cause a ground loop.
Anything that induces a yaw away from the longitudinal alignment of the CG & centre of drag from the gear can precipitate a ground loop.
This can include:
A tricycle a/c can behave like a tailwheel type if it has a relatively light load on its mainwheels & a relatively heavy load on the nosewheel eg if you wheel barrow by landing on the nosewheel, or if you try to force it to stay on the ground too long during the t/off roll.
I have a feeling I've forgotten something in the above...
[This message has been edited by Tinstaafl (edited 11 October 2000).]