He has been unaccountably delayed.

他被莫名其妙地耽误了。

《牛津词典》

And then, unaccountably, she giggled.

然后，莫名其妙地，她咯咯笑了。

《柯林斯英汉双解大词典》

Animals are also affected, particularly before earthquakes, snakes have been observed to come out of hibernation, rats to flee from their burrows, dogs howl and cats jump about unaccountably.

特别在地震前，动物也会受到影响，蛇被观察到从冬眠中醒来，老鼠从洞穴中逃走，狗会狂吠，猫会莫名其妙地跳来跳去。

youdao

Things improve a bit when the tough, direct Jennifer Lawrence shows up as a young neighborhood widow who unaccountably pursues Pat.

NEWYORKER: Silver Linings Playbook

Unaccountably, he decided to hang on until the next year despite signs that the unions could no longer deliver wage restraint.

ECONOMIST: Obituary

But no one knows what was going through his mind when his car unaccountably left the road and crashed into a eucalyptus tree.

ECONOMIST: Joseph Wolfson