Why can excessive use of the trailer hand valve (the separate trailer brake control) increase skid risk?

Correct answer to question 107 (Combination Vehicles): Applying only the trailer brakes hard can cause the trailer wheels to lock and the trailer to swing out

Why

Using the trailer hand valve too hard applies braking force only to the trailer wheels, which can cause them to lock and the trailer to swing out, so it is meant for occasional use, such as checking a swaying trailer, not as a primary brake.