One of the commonest reasons for the loss of a tribe is an outbreak of sickness. If your farmer/s cannot work, the tribe will starve; even if you have other villagers with farming skills they will not switch jobs to save themselves. It's important to have doctors to heal the sick in such cases.

It's probably too late now, as it has probably been overwritten, but your log file should give a clue. See the sticky thread about the log at http://www.ldwforums.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=55432&page=1#Post55432