Lurching after turning engine off????

[WestCoastSVX]

This seems strange to me, but I'm sure someone will know and it will probably be bad news.

The last couple days when I turn off the car, it suddenly lurches back an inch or so - enough that you really feel it sitting there in the car.

My usual procedure is to stop completely in a parking space, put the transmission into "P", let off the brake, put on parking brake, open my door (because the auto seat belt only works while the engine is on), then turn off the engine.

You can see from that order of events, that all brakes have been applied and released by the time I power down the engine and the car shouldn't (and never has before) move.

But now on that final step of turning off the engine, with the car in park and parking brake on, it "lurches" back a bit.

Very disturbing!

Transmission related?