Inexplicable (adjective)
Impossible to explain; not easily accounted for.
Unexplainable (adjective)
That doesn’t have a known or readily available explaination.
Wiktionary
Inexplicable (adjective)
unable to be explained or accounted for
“for some inexplicable reason her mind went completely blank”
Unexplainable (adjective)
unable to be explained or accounted for
“he was subject to unexplainable rages”
Oxford Dictionary