Hi,
Everything is set to automatic. What I've found out is that DRS is obviously allowed to violate HA failover. If some number of VMs are powered on, but has very low load, DRS will all put them onto as little number of ESX hosts as possible. Then, when those VMs are under heavy load, DRS will migrate them to other ESX hosts even if HA failover is violated. I think this is wrong or at least there is no setting to overcome this.