So twice now while fighting Great Buffalo we have had him despawn mid-fight. Basically what happens is:

1. Someone dies.
2. Buffalo does Sonorous Blast.
3. Person returns to crystal.
4. Buffalo attempts to chase this person and then despawns because claim is lost and it's now out of the spawn range.

The people I play with are assuming this is because death does not erase enmity generated and once Sonorous Blast has gone of it wipes it off the people who should be keeping it (Tanks/Off-Tanks). Because of this it goes after the person who has returned because they now have the highest enmity generated.