Why does a pendulum move as if it has a mind of its own

[abar45]

My teacher for Physics hns proposed an extra credit bonus if anyone could answer this question dealing with a pendulm. Why does the pendulm move as if it had a mind of its own. Refering to the circular motion the pendulm starts making after a couple seconds. When the pendulm is released to go in a back and forth motion (a straight line) it starts off straight but then starts to make a circular motion, usually counter clockwise. What is the reason in lamens terms for it's circular path. Please!

 

[Integral]

In a Lab room pendulum such motion is due to small motions of the mounting bracket and/or non-uniformities of the mount. I suppose if you are using a wound string it could cause a torque that would add components of motion causing the circular motion.

Now if you have a very long pendulum mounted with a very ridged mounting you will be able to observe an apparent motion of the swing path caused by the the rotation of the Earth (http://www.phys.unsw.edu.au/PHYSICS_!/FOUCAULT_PENDULUM/foucault_pendulum.html" . This is not related to the circular motion you see in the lab.