After Effects added an extra move to the animation, how to get rid of it? As you can see in the Value Graphic, the second part of the line is a broken line, but I didn't make it that way, and I can't fix that.

That causes the shape to continue moving forward a little bit instead of going back immediately.

I didn't set easy ease or staff like that. It happens every time, but I don't know what is controlling this!

Help!
OK, I finally figured it out. This bug is known as "Boomerang Effect".

Reason: Bezier will cause After Effects randomly mass up the path of the shape. 



So you can simply right click the keyframe than select "Keyframe Interpolation" from the menu, than set "Spatial Interpolation" to "Linear" instead of "Auto Bezier", it'll fix everything!

These two video tutorial helped me a lot: AE Boomerang Effect, Part 1 and AE Boomerang Effect, Part 2 they explain the principle of this problem.