You may have seen the play in question. Daunte Culpepper drops back to pass, but there's a heavy rush. He rolls out to his left to evade the would-be tackler. But, he also moves a little backward, in sort of a semi-circle move.
Anyway, the rush catches up with him, and he heaves the ball out toward Chris Chambers, who is a few yards past the line of scrimmage.
The referee stops and evaluates the situation, and then drops a flag, calling intentional grounding. But, what happened? Daunte was outside the tackle box. Chambers was clearly "in the area." But, the pass came down at about the 20 yard line, and the line of scrimmage was about the 22. So, the pass didn't make it back to the line, and therefore, the penalty.
Quite correct, and a nice job by the official.