If this was experienced while the rider was receiving pushback from the board, it can be common for riders to counter pushback by unconsciously shifting their weight forward to bring the board back to level. By leaning forward, you are requesting that the board accelerates more, and stop the board from decelerating. That is why it seems like you can't slow down.
If a rider is fighting pushback or is experiencing extreme pushback from the board, the board will tip the nose up and lower the back and does not force the board to brake if the rider keeps their weight forward. The best way to handle this is to relax your hips and allow the board to push your weight back and slowly brake. If you continue to fight pushback and feel like your tail is too close to the ground to brake, just remember that you do need to shift your weight back to brake and only a small amount of angle change is necessary to brake and you can brake safely. The best way to avoid braking with what feels like not enough room is to relax when you feel pushback as the board will naturally shift your body into the proper position to slow down.