This happens far more times than gets reported! Always costly and the biggest fear is that ground crew might get hit, they come out at quite a speed.
Crew opening doors is partly an OTP reason along with the hassle of getting all the handling agents to have different procedures for different airlines becomes a headache. Disarming whilst moving is also because as soon as the Aircraft stops, pax get up and start moving about. Giving the crew a few moments of space to concentrate while pulling onto stand is supposed to reduce events.
There's a large amount of human factors behind events like these. Like pulling the pin out with the same arm and same way every time. Dis-arming motor memory discipline.