Fantasy trade value is ever in flux.
Injuries can have an effect on a near daily basis. Christian McCaffrey practicing Wednesday has his trade value back amid the elite. David Montgomery's knee injury makes him one of the players whose trade value took a dive.
Injuries to a player's supporting cast can make a major difference too. You always have to consider adjustments to a team's depth chart, whether the schedule is hitting a rough or easy stretch, the trend of a player's recent performances, etc. Trying to forecast the best players to target, and the worth of the players you roster, is an ever-changing and inexact science.