Why we have to disable HA before "Force to Standby"

What version are you running? What is your HA mode set to? Load Aware, HA Order, etc..

If you can share the portion of your configuration in the /cm section that shows your HA features, that would be helpful.

If you have HA groups configured incorrectly, if you have a preferred active device set up, or the default load aware failover mode could cause this.

I'd try changing the failover type to HA Order if you're not using it now.