"Sync all cores" is a BIOS setting that applies the same multiplier to all 6 cores. The alternative is to apply a different core clock multiplier depending on the number of cores active. BIOS does this before entering the OS.
I think the activity you see in Task Manager occurs as Windows activates all its drivers and default services. Even when there are no applications running, Windows opens and starts many of its internal services. Some application processes start automatically, such as antivirus software. See the Processes and Services tabs in Task Manager. All those shown as "Running" were started by the sort of activity you see just at OS boot. Activity drops to 0% as it should when all that initialization finishes.
Jeff