Because alot of people aren't satisfied with their drafts. They feel they are still weak in one area, or they made a big mistake. For example, I thought I could get Larry Johnson late in my draft (I took Priest) and I was wrong. So I traded Dunn after the draft for him. Some people also mistakingly take players with the same bye weeks and may look to improve their teams that way.
Some people wait too long and miss out on a good QB or RB, and they stack up on WRs to trade after the draft. It's not that uncommon. Not many teams are perfect after the draft. Every team can be improved.
QB- M. Vick
WR- J. Jones, M. Colston
RB- R. Rice, J. Charles
TE- A. Hernandez
FLEX- D. McFadden, F. Jackson
K- G. Zuerlein
B- T. Romo, J. Kerley, K. Hunter, V. Ballard, D. Thomas, B. Myers, S. Suisham