After the a/c rolls it starts to sideslip, causing the resultant airflow to be slightly offset from the fuselage centreline (the longitudinal axis). The fin causes the a/c to yaw into the now offset airstream.
Meanwhile the lower wing, being swept, experiences a significant increase in its lift producing ability compared to the other wing. This causes the a/c to roll back upright. Inertia causes the a/c to continue rolling so now the effect is repeated on the other side.
End result is a series of roll - yaw - roll upright - continue to roll - yaw the other way - roll back upright - continue to roll etc etc. although it can be a very slow cycle.
The nose of the a/c will seem to trace a horizontal figure '8'.
I think it's caused when an a/c has greater stability in roll compared to yaw. The reverse would cause 'snaking'.