6 year old afraid of taking a shower?

my little sister is 6 and shes been afraid to take a shower for as long as i can remember!! so today out of curiosity i turned on the shower while she was playing around in the water during her bath. she completely started freaking out and ran out of the bath tub and wouldnt stop crying for 2 hours. she is so scared of the shower. i dont understand how a 6 year old could be afraid of taking a shower? i feel horrible that i did that but i wanted to see how terrified she was of it. how can she over come this fear of taking a shower?