Someone forgot to lock the back door.
No matter how awesome you think your zombie fortress is and how impregnable you think you’ve made it…. The Zombies will get in, and they will get you.
If it’s not the zombies themselves getting in, then your fellow survivors will inevitably do something stupid and boom… You’re all dead. Have you ever seen a zombie movie where something like this didn’t happen? Of course not, if something doesn’t go wrong there will be no conflict and therefore no story.
That’s why my zombie plan involves immediately locating the weak links in whatever group of survivors I find myself…. and quietly doing away with them. That’s merely self preservation.