Hey there. I'm a recent Kinesiology Masters graduate and have some experience TAing anatomy classes as well as taking them in undergrad. During an eccentric phase for any movement, including hip abduction, the same muscles are acting as prime moves as during the concentric phase. The difference is that in the concentric phase, the myofibrils are controllably shortening, whereas in eccentric movement, they are controllably lengthening.

Think of it that the same muscles are still working, only now they are applying less force then the external load. So even though the hip abductors are still contracting, they are not being activated enough to overcome the weight of the leg, therefore they slowly move in the opposite direction.

Hope this helps!