Why did my 5 week old baby bunny die when Mom still in cage with 2 others Lethargic and listless for a couple of days and sat alone with eyes half closed weird mucusy fecal matter stuck to her bottom?

Your bunny had diarrhea, possibly due to a bacterial or viral infection. It is likely that your bunny became so dehydrated from the diarrhea that her internal organs shut down.

In the future, if your rabbit starts getting listless and stays by herself in the corner, you should take her to the vet immediately for treatment.