Its possible the guys are descending on profile using VNAV and theres a speed/height constraint plugged into the FMC telling the automatics that 250kts below 10000' is required; meaning the plane would lose the speed before descending beneath the 10000 restriction(you can place the speed/height restriction in yourself) ......but you say 8000 so possibly not
EADI
don't fly a 737 so only looking in from the outside hazarding a guess
