Visitor Visa renewal refused

[dr_rb]

My mom had 10yr visa before which is expiring in Aug 2018. When I started the renewal application with DS-160, it gave me an interview waiver confirmation letter and just instructed me to submit the confirmation, her passport and photo through dropbox at Kolkata. I did that but they called her for an interview. when she last came here in US in 2016, I extended her visa and she stayed for a total of 11 months. While applying for the renewal, in the Ds-160, I mentioned that, but there was no scope to elaborate. So when she was called for the interview they asked her about the last stay ....she showed them the visa renewal notices too..also the letter of request to renew that I sent to USCIS here. But they still gave 'administrative cause' and said visa is refused and they said to send them clarification by email on why she stayed 11 months last time along with the proper copy of the notices. I am thinking of sending them a detailed letter explaining. Is there anything else I should send other than the relevant documents? Should I need legal help in replying. I am a US citizen. Please help on my next steps.