I trade before the first game sometimes and it isn't all the unusual. I look for the following situations and pounce....
I often look for trade value especially in picks 8 and beyond. I tend to to end up with wealth at a position. Then I look for the guys that weren't paying attention to what they were doing. For example in 1 leauge, a guy had Burress, Curtis, and Bruce at WR. I happen to have some nice WR's and want to upgrade at a position like RB. He has a couple of decent RBs on his bench, so I offer a trade. Since he is under pressure, he is forced to trade a RB he took in the 6th for a WR I took in the 9th. Great deal for me.
I also look for guys I thought would drop further but didn't. For example one trade this year was for my backup TE. I thought LJ Smith would be there in the 16th but wasn't. He went in the 12th. I traded a late pick WR to the owner for him. Sure, had I known he would go in the 12th, I would have took him, but I gambled and lost. This gives me the guy I want even though I missed him in the draft.
Honestly some guys don't really know what they have, and prior to the first game is the perfect time to try and upgrade. If LJ Smith gets 80 yards and a TD because the other WRs are hurt in game 1, I got zero shot at getting him.
Doesn't always work, but thinking you can't upgrade from who you drafted prior to week one is just loser think.