Derpy’s just played her Cannon to file 5 for check (with the Advisor right in front of it to create a line of fire for the Cannon). Pinkie resigns because she lost a tempo or two she needed to salvage a draw, and checkmate is just a matter of time. The Generals cannot face each other on an unobstructed line (nor can they leave the Palace, the 3x3 square denoted by the diagonal lines), and even interposing the Pawn (as the Cannon cannot overleap two pieces in one move) will only delay the inevitable.
 
I figured Pinkie would be a good sport about losing.