There is no "Hard Mode" in this game, so perhaps you mean "Story Mode"? Either way, there is no difference in the daily mechanics between "Story Mode" and "Sandbox Mode," so you would have the same problem either way. There isn't any singular cause that can reduce obedience by 50 points in a single turn. There are some random events that can reduce obedience by 40 if poorly handled, but that would only affect a single slave. My best guess is that you have not cleaned your mansion, which can add stress and reduce obedience. And high stress can also reduce obedience, so you can get a double hit to obedience from not keeping the mansion clean which can add up to a loss of 50 obedience per turn if you are unlucky. Both of these problems leave messages in the End of Day reports for the player to read. Additionally, if you have high grade slaves, then they will have luxury demands, which if not met will result in a daily loss of obedience and a message in the report saying that they are unhappy with the quality of their living conditions.
The fastest way to improve the obedience of your slaves is to use the Meet or Sex (if you have their consent) interactions as the better they feel after the interaction, the more obedience they will gain. It's quite easy to max out a slave's obedience in 1 to 3 interactions depending on their initial conditions, but that won't help much if you don't improve their living conditions.
Also note, don't have multiple slaves sleep in the master's bed unless they all have had sex together. If they don't like each other, then they will really stress each other out.