Maybe ask this in support - but my guess is considering the time was so short, you had actually lost on time at that point, even though the clock didn't show it. The shown time in live games is not exact - because there are adjustments due to lag and such.
Just a guess - when I run the game through the analyzer interface I can make that move, but I don't have a clock going either.
http://live.chess.com/simple?v=2015033001#g=1105710337
In the game above.. after 52..Rc2x... I still had about 8 seconds left, I was trying to move King to b6.. would not allow... nor any B column.
And yes I know I lost either way... but the point is.. I should still be able to move my King into that column and it wouldn't allow me. This only ever happens when playing people above 1900 also... what's the deal? How about a fix chess.com.