Just got a Yaris - AT downshifts when braking?

[Jlager]

I just bought a 2009 Yaris 5-door hatchback and have noticed something that seems odd to me but maybe it's completely normal.

When driving at highway speeds going down a fairly steep hill if use the brake, it seems as if it downshifts into 3rd as soon as I touch the brake. This causes the car to jerk a bit. It isn't bad, just something I've never experienced driving an AT before.

Is this behavior by design (engine braking to assist with the braking?) or is there something wrong? It doesn't seem to happen if I'm driving on flat road at highway speeds and use the brake, only when going downhill.