Hi,

Thanks for your question, and I'm sorry you're seeing a change in a plant that has been a steady grower for six years.

I think you can rule out location as a problem, for now, since the plant has been happy up to this point (dieffenbachiea need sufficient light and prefer temperatures between 65F and 80F).

You are right to ask yourself if the plant is getting too much water, since that's a common source of plant trouble, but I would resume watering when the top 1"-2" of soil is dry. 

You don't mention any pests.

Is it possible the plant needs to be repotted? Cramped roots and poor water drainage could stress the plant enough to drop leaves.

Lastly, the plant is six years old. It's normal for the plant to shed some lower leaves as it continues its growth upward.

In summary, consider transferring to a slightly larger pot, go back to watering when the top 1"-2" of the soil is dry, and make sure the plant is getting the light it needs without any cool drafts or blasts of warm, dry air. Write back if you don't see an improvement.