Why would an Army BCT ship date suddenly change, not just for me but also for others?

Why?

Why?!

Welcome to the Army! That's why.

Don't worry, you'll acclimate.

My platoon/section motto in Korea was "If it wasn't for the last minute, we'd never get anything done" because of all of the changes.

This sort of thing happens all the time. Just wait until you're coming home from a deployment and you tell your family it will be on a certain date only to have that date pushed to the right. And then again. It happens. Get used to it.