Odds and lines can change for a variety of reasons. The most prominent one is “Change in circumstances”.
The initial odds and lines set by the odds compliers are based mainly on the probability of possible outcomes. With that in mind they have to take into account all various factors that can affect what is likely to happen. If any of those factors change for any reason, the relevant odds need to change.
Take a football game for example. When setting the odds for each team to win, the odds compliers are going to consider a range of different factors. The most significant is simply the overall quality of each team. If one team is clearly performing better the odds of this team winning are going to be quite low.
But what if their three top players get injured the day before the game? They’re no longer quite so likely to win, and this will eventually be reflected on the odds. The odds on the winning will go up, while the odds on the other team will go down.