Main Cause
Primary Cause
noun
The main cause is the main cause or the primary cause.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
下りの主因はおそらくさっき食べたソフトクリームです。
The primary cause of my diarrhea is probably the soft serve I ate just now.
ニアミスの主因は、この電車がスピードを上げるべきではないタイミングで上げたことです。
The main cause behind the near-miss is that the train sped up when it wasn't supposed to.
癌や自閉症など、多くの病気の主因は未だに分かっていません。
The main causes behind many conditions like cancer and autism are still unknown.