What would make you play Nc7?
It is the only move with which White can give check mate in two moves.
This results in checkmate in 2 if, as the computer does, Rxe5 is played, but why wouldn't the computer just play Nxg5, winning the queen and preventing checkmate?
On 1. ...Nxg5
White plays 2.Rd5
which check mates.
Or d1 and promote to a queen?
On 1. ...d1Q
White plays 2.Rd5
which check mates.
SUMMARY:
1.Nc7
has been played to free the rook on e5
to give check mate, by coming to d5
.
This is why computer played 1. ...Rxe5
, since on every Black's move 2.Rd5
check mates, except for these:
1. ...Ne3
but then White mates with 2.Rxe4
.
1. ...Nxe5
where White mates with 2.Rxd6
.
1. ...Bxe5
where White mates with 2.Qxd2
.
1. ...dxe5
where White mates with 2.Bc5
.
1. ...Kxe5
where White mates with 2.Bg7
.
Best regards.