(According to the rules, the offense must have 7 on the line of scrimmage.)
Dez needs to know where he's supposed to line up. The penalty was called for two guys moving at the same time because Miles was in motion when Dez took a half step forward. Dez took that half step because either he realized too late that he had to be on the line of scrimmage for that play, or he was confused about where the line of scrimmage was.
Watch the vid of the penalty in post#64. Curiously, just before Dez moves, he looks at the line judge (the guy who threw the flag two seconds later) and points at the sticks. Twice. Unless the line judge is trying to communicate something to him, I have no idea why he's looking outside instead of looking inside at the football. It's like he's trying to tell the chain crew that the LOS marker is in the wrong place or something. In any case, he's extremely unsure of where's he's supposed be standing, and he probably could've just stayed where he was without a penalty, based on how the other receivers were lining up in the game.
In Dez's defense though, the officials were not consistent with this penalty.
At 0:16 of hairic's video that follows (from early 3rd quarter) Dez does exactly the same thing--only in reverse--while lined up on the other side of the field, but no penalty is called. Since Miles is at the LOS, Dez has to back off one yard to avoid a penalty. Watch Dez look toward the sideline again as he backs up. Notice that while Dez is stepping back, Witten is also moving. This is the same official who was looking right at all of this and kept the flag in his pocket.
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