un ‧ ex ‧ plained /ˌʌnɪkˈspleɪnd◂/ BrE AmE adjective
[ Word Family: adjective : ↑ unexplained , ↑ explanatory , ↑ explicable ≠ ↑ inexplicable ; noun : explaining, ↑ explanation ; verb : ↑ explain ; adverb : ↑ inexplicably ]
something that is unexplained is something you cannot understand because you do not know the reason for it:
patients with unexplained symptoms
For some unexplained reason, he wants to move to Ipswich.