He said if you watch the video, you'll see that Lazar was three feet behind the end line. When he ran the to the other side he had to go around the standard that holds up the basket. That put him right on the line and when he attempted a throw in, he took a step forward (as you normally would) and stepped on the line.
He wasn't making a excuse rather explaining what happened. In other words, in the heat of the moment, going around the standard put him to close to the end line. It wasn't that he "just made" a bone-headed play.
I also happen to believe it was more of a physical mistake than a mental mistake. He was going to Acker, and Carrol simply 'jumped the route' which Lazar reacted to and caused him to double clutch/pull it back. If he lets it go, its picked off easily and the game's over. Mac's comment is interesting, because obviously, ideally he is further behind the line and is able to pull it back and call timeout (not sure if they had one left, but I think so).
Ah well.
Here is the play
http://www.youtube.com/watch?v=NpnUwzUCGF0
2:10 into the video.
Mac is right, you can see Lazar go around the standard and move closer to the end line. You're right he was attempting a pass to Acker so he stepped forward. Going around the standard put him too close to the end line and he stepped on it as he tried to pull it back.
Boy its a shame too, but give credit to Carrol and Mizzou, because if he gets that thing to Acker, MU seemingly has a 3 on 2, with Acker flying down court (almost at half court already) and McNeal and Lazar trailing to crash the miss. I would say that may be another reason Lazar stepped over...I think he was anxious to get to the other end as quickly as possible.