While routing I sometimes come across this strange and irritating phenomenon. This usually happens when I save the pipe drawing as pdf and then I can't fix it. I usually have to redraw the pipe and this takes up quite a bit of time.
The route alignment changes by itself or one of the fittings, usually the one on the starting point, just moves to one side and floats on its own. The starting point on the model is still at the initial flange but the rest of the pipe is misaligned. See the attached picture.