Aggessive Bee

I have a large clump of Teucrium germander which is visited annually, when in flower, by a solitary Wool Carder Bee.

The insect is very aggressive and attacks any other bee which visits the plant by knocking it to the ground, or in some instances, appearing to smother it for a few seconds. The victims soon recover and move to a different part of the plant to feed, but the Carder bee pusues them until they leave.

I've watched this behaviour over the past few days, the same attacks happen continually, and the Carder bee constantly patrols the Teucrium.

Today, however, it developed a new pattern. It suddenly flew off the Teucrium to a Stachys 'Hummelo' (a distance of about 10 yards away0 and knocked  a large bumble bee to the ground before returning to it's Teucrium.

Can anyone explain this behaviour, please?