"key reason behind" is correct and usable in written English.
You can use it when you are talking about an important rationale for something. For example: "The key reason behind the company's decision to downsize was to reduce overhead costs."
These growing layers of complexity are a key reason behind the still sluggish current account switching market.
Poundland has pointed directly to the fact there are fewer people on the high street as a key reason behind its slowing sales.
The midfielder said Guardiola's convincing philosophy was a key reason behind his decision to swap the Bundesliga for the Premier League.
North Korea cites what it perceives as a US military threat as a key reason behind its need to build nuclear weapons, and has poured a huge chunk of its small national budget into defence, science and technology.
This inconvenienced Grapple planning, but, on a larger scale, underlined a key reason behind the operation, the schizophrenic nature of the Anglo-American relationship, which had peremptorily squashed the Suez adventure.
In MSM, higher numbers of partners is a key reason behind the heavily inflated rates, compounded by the use of apps such as Grindr, venue based and group sex.
Key reasons behind that: the tough economic times that the US and Europe are living through.
Twitter's openness has been praised as one of the key reasons behind its meteoric growth.
One of the key reasons behind the successful operations of the Chindits was the support they received from the local population against the Japanese forces.
Scott Parker has identified Aaron Lennon's form as one of the key reasons behind the run of results which has taken Tottenham to third in the Premier League.
The information and communication technology (ICT) and other few factors are the key reasons behind the technological progress.
