Licorice contains a substance called glycyrrhizic acid. This causes the kidneys to flush potassium out of the body. Digoxin is very sensitive to potassium levels; when potassium is low, digoxin binds more strongly to the heart muscle, leading to digoxin poisoning (toxicity).
This is a companion discussion topic for the original article at https://patient.info/medication-interactions/digoxin-and-licorice-interaction