背后原因英语翻译怎么说

"Behind-the-scenes reasons" or "underlying reasons" are both common ways to translate "背后原因" into English. You could also say "the reasons behind it" or "the root cause" for a similar meaning.
For example:
"Can you tell me the underlying reasons for the decision?"
"There are some behind-the-scenes reasons that we can't disclose right now."
"I'm trying to understand the root cause of the problem."
"What are the reasons behind it? I'm curious."