I know its brief but why exactly does abnormal septal motion occur in the heart other than septal defects?

Several things: It can mean that the pressures in the heart are abnormal(volume overload), abnormal conduction(like bundle branch blocks) of the heart muscle, abnormal muscle development/function in the heart(like to much muscle so called hypertrophy), and valve replacement are just some of the reason for abnormal septal motion. Hope this is a reasonable start for you.

Septal defect: Hi, r w talking about Ventricular septal defect or ventricular septal defect? 2 different entities with different etiologies. please clarify and ask again, Thanks

Refersto wall motion: Abnormal septal motion on an echocardiogram typically refers to abnormal systolic motion of the septum, which can be from a bundle-branch block on ECG and/or ischemia. Typically bundle branch block can be verified on ECG easily/or on the Holter monitor you already had. Given your young age, ischemia ought not to be the case.