I saw some people discussing D5 as a space on a chessboard as well. I think Cboldt was part of that.
Thank Q for the ever growing lexicon!
D5 is a space on a chessboard, but in the Q context the reference was speculated to be part of a certain set of chess moves in a famous game.
I'm of a mind that in the context of that Q drop, D5 is a reference to the avalanche danger scale. "The snowball has begun rolling - there is no stopping it now. D5."