This what a Spline Curve does in Poser.  In the above picture each of the Keys is at Zero except for the one with the green line (frame 75) it is at -100.  If you got rid of the Key at 75 then the line becomes flat.  Poser will draw a line between all the points and make the "Best" possible curve to do it with.  Even though it is not what you want. The Easiest solution is the use just Linear Lines.  And the easiest way to that is to collapse all the figures on the animation palette and drag a selection box around all of them and then click the Orange Linear check button to change all Keys to Orange from Green.  Th downside is that all movement will be at the same speed, however you can put in two extra keys one just after the movement starts and the other just before the movement ends and place them so that it starts slow and ends slow.

Putting in extra keys is something you are going to have to do no matter what, so you might as well use a method that will give predictable results.  The other thing is it is best to use the pose dials to make movements and not the Pose window. The dial will only affect a single channel whereas the Pose window will affect dozens of channels.  Th Pose window is best for static shoots, where you don't care how the parts get into place.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13