When you get up from bed without turning, you are putting maximum stress on the muscles of anterior Abdominal wall. If you are used to getting up like that when you are not
pregnant then nothing happens because your Abdominal wall muscles are always in that state of tone.
But if you are not used to exercising your Abdominal wall then a sudden strain is put on Uterine wall which can lead to pressure on the placenta, even precipitating haemorrhage at times.