i believe that is "en pass ant"
pawn moves two squares to avoid being taken. Opponent can capture diagonally since their pawn is on the 5th rank.
It does not happen often but is perfectly legal.
http://www.chess.com/learn-how-to-play-chess.html#special
(taken from the above)
En Passant
The last rule about pawns is called “en passant,” which is French basically means “in passing”. If a pawn moves out two squares on its first move, and by doing so lands to the side of an opponent’s pawn (effectively jumping past the other pawn’s ability to capture it), that other pawn has the option of capturing the first pawn as it passes by. This special move must be done immediately after the first pawn has moved past, otherwise the option to capture it is no longer available. Click through the example below to better understand this odd, but important rule.
my last play vs nasticore
move 44.a4bxa3
not legal !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!