The meaning of waiting for the other shoe to drop is to "await a seemingly inevitable event".
Waiting for the other shoe to drop comes from the time when a person who lived above you in an apartment and they would take one shoe off and drop it on the floor which would keep you awake.
Then it could take a few more minutes for the other shoe to drop on the floor and make the sound.
So you would be waiting until the person dropped the other shoe so that no more noise would be made and then you would go to sleep.
The expression of waiting for the other shoe to drop came from the 1900's era when a person was awakened by a neighbor who loudly dropped their shoe when they lived above them in the apartment.
Waiting for the other shoe to drop just waiting for something to happen that is gonna happen whether you want it too or not.