Let's first take a look at the motion of point D. This motion of D is shown in the simulation results below.
The motion of D is circular, with the center of the path located at point A. This is expected since link AD is pinned to ground at point A.
Now, let's attach an observer to link BE. Keep in mind that this observer is unaware that they are moving. The motion that this observer sees is straight, with this straight path aligned with the slot cut into link BE. You can see this is the following animation shown from the perspective of the observer attached to link BE.