What would cause my AC to drained out of the overflow pipe?

;) It's supposed to.

That pipe is for the drainage of condensation.

By overflow I'm assuming you mean its not draining where it is supposed to?

The drain may be clogged or your evaporator may be frozen, check and repair these conditions as needed.

Thanks for the advice.

Now you've got my curiosity up. Was it just that you were wondering if water is supposed to be dripping from the drain line coming off the furnace... or is there another issue?