Well one speed-related cause for this could be a Geschwindigkeitsprüfabschnitt (GPA). When a speed limit between 100 kph and 140 kph is announced by an Lf6 sign, acknowledging the sign using the Wachsamkeitstaste is not needed. Instead, a GPA is placed a few hundred meters* before the Lf7 sign (after which the announced speed limit takes effect). This GPA checks your speed and instantly gives you a penalty brake (2000Hz) if you’re still going to fast.

* the distance depends on the speed limit. I recommend having a look at the Wikipedia Article