The HSB pin is a bidirectional pin. When it serves an an output pin it can
1) Indicate that the nvSRAM is busy in executing power on boot up
2) Indicates that a STORE cycle initiated by the software or the hardware pin is in progress
3) Indicates that VCC has dropped below the VSWITCH
During power down as the 3rd point criteria is met it will be internally pulled low. It is normal behavior.