The "disaster plan" that congregations are encouraged to have is in response to Hurricane Katrina several years ago.
Immediately afterward, due to relocations, several thousand JWs were unaccounted for, and the elders of local congregations couldn't figure out how to find them.
Of course, it was the perfect opportunity for those who wanted to "fade" even more quickly, and I expect a large number of those who couldn't be found, didn't let themselves be found on purpose.
The WTS doesn't want a repeat of that scenario, so they send reminders several times a year that each congregation should have a "disaster plan"; the elders should maintain a list of each publisher's multiple phone numbers and an out-of-state contact, etc.
Of course, many congregations have elders who are scarcely competent to tie their own shoes, so the compliance to this "disaster plan" policy is far from 100%.