Forking the enemy queen and knight in order to force a queen exchange, thus pre-empting a potential white attack.
Preferable was 5.Nc4 Rd5 6.Rd1 Ra7 7.Rxd5! exd5 8.Ne3! with chances for a draw.
1...Qxc6?! 2.Rc1 Qd7 3.Qd4 , followed by 4 Rfd1, the d5-pawn turns into a problem for Black.