(( Say aliens were attracted by the disturbance in the time space continuum , aka time travel ))
From there you can have it so your vault is
A.) Alliens attack vault - super overpower the aliens more than even the current deathclaws
B.) The time travel causes Mass radiation damage on all dwellers dropping them to a low health (3/4 radiated)
C.) Vault is locked for the amount of time you went back in time.
D.) Aliens Abduct all dwellers and your vault dies.