Well there is two possibilities and one is a safety precaution to keep from ruining the washer .
if the washer cannont distribute the load evenly it will only go into a slow spin try adding more clothes.... to evenly coat the drum >>. the other is if it is out of balance or the wsher isnt't level it will shake violently and spin in slow cause it detects an imbalance problem if this machine has operated a long time overladed , or unlevel it can wear out the tub schocks which takes all vibrations of the tub to give more of a gently ride the a are like schock absorbers....