There are several reasons that could cause a massive change of the odds for a specific event. A last minute injury of an important player, a dramatic change of the weather conditions could also considered as an important reason, some wrong estimations by a bookmaker as well, however an important reason could be the total amount of stakes placed on the event, which may indicates that the match is fixed.
You should always keep an eye on those dropping odds before making your decision as they may hiding important facts. This page displays a list of events that their odds have recently changed. Click on the event to see information about dropping odds from a number of bookmakers.