Rek'sai gets snared; still has movement

I was playing a solo queue (ranked) on Summoner's Rift, just recently, and noticed something extremely frustrating and odd. I was playing Lux mid against a Riven mid, and a Rek'sai jungle. When Rek'sai came in for a gank, I snared both Riven and Rek'sai, however, Rek'sai continued to move towards me. She was burrowed, didn't ult, and didn't use any other abilities that I could see. Can anyone give me clarification as to why she still moved after I snared her? Her abilities don't mention anything about immunity to cc, and it's very annoying. Thank you.