looks like the fan had a cracked blade..maybe from a bad install..from a used one or instaling and coming contact with some thing.. the crack wouldnt show. but the pic showed were it was cracked .and then the give away crack.and the sound you heard might be the blade coming off as it turned..and that vibration of a missing blade would shake and maybe just maybe.. cause the bolts that were on to tight or to loose..would bend or do what happened..
thats just my guess..

sorry ..
is it me ..does the pictures seem very big
????