Dropping Odds means, the people who are betting so heavily only on one side of the bet. If the line is opened at the certain price and then suspensions, injuries and other factors will impact the potential game, the price on that game will drop. You need to know the example if you want to understand the meaning of dropping odds and you can choose the example from UEFA Champions league.
In round of 16, Neymar from PSG was injured when his team played against PSG. The line for the Red Devil was moved down from +300 to +200. When United lost the game on the first leg 2-0 at their own home, the line moved to the opponent which is PSG to -10000. Why people bet on PSG more than United? People know that United is the major team and they are one of the biggest team in English Premiere League. However, when people knew that United lost in their own home, many people lost their faith on them.
It means, people hesitated to choose United because they didn’t want to lose their money when United lost again. On the first leg of round 16, United lost at Old Trafford. It means, on the leg 2, United might lose again since they would play at PSG’s home. When PSG played in front of their home, they would attack United so much and it resulted United to lose and they get eliminated from Champions while PSG made it through to the round of 8. This is the reason why odds are dropping. It doesn’t mean that other people will not choose the opponent. Some people might still believe at that time that United could win.