Buckler Block: Building up stun bar

When passively blocking, the buckler is building up the stun bar eventually leading to back to back heavy stuns.

I thought it was due to my shield being raised but it seems to happen consistently. Regardless of zone or act. This seems exclusive to bucklers and not shields in general.

Edit:

Found a possible connection, stun builds up during Rapid Assault skill gem use and if stun bar is filled I am heavy stunned. If I use rapid assault again, an enemy striking me will trigger another heavy stun.

Rapid Assault does not state to count as blocking or raising the shield.