The details of the principal cause are not known.

主因未详。

《新英汉大辞典》

The climate is changing, the effects are already being felt, and human activities are a principal cause.

气候正在变化，已能感觉到其影响，而人类活动是一个主要原因。

youdao

Apart from relapse of a person's disease, GVHD is the principal cause of death in patients who have received donated bone marrow.

除了患者病情复发，移植物抗宿主病是接受骨髓细胞移植病人死亡的主要原因。

youdao

Stormwater runoff, caused by rainwater running off impervious pavement, is a principal cause of urban waterway pollution nationwide.

FORBES: Smart Communities will Build Green Infrastructure

Property rights, she says, could not have been the principal cause because England and many other societies had stable and secure property rights for a long time.

WSJ: Cultural Conversation: The New Theories of Moral Sentiments

Other supporters of the prime minister are calling for Mr Fini, the principal cause of Mr Berlusconi's weakening grip on office, to resign as speaker of the lower house.

ECONOMIST: A lively August for Italy��s politicians