From a standing start, gimbal pointed straight down, if you start off the gimbal can compensate for the pitch of the drone as it starts to move forward because the gimbal moves away from the AC body. When you stop suddenly and the AC pitches rearward to "brake" the gimbal movement cannot compensate because it is at the end of it's available movement and on the video it will appear the gimbal has jumped "up"...why it remains there is for DJI to answer. If you very slowly come to a stop the AC will not pitch as far, if at all, and video will remain smooth. The same will happen if you are at a stop and suddenly move in reverse, the gimbal will jump up because there is no further available travel when it is at that limit.
Also be warned, when the gimbal is pointed down at 90 degrees, performance is limited in video mode, unless the AC is set to Sport. This can lead to decreased wind resistance and a blow away. Another DJI mystery feature.