I'm fairly new to the game, so please bear with me on what may be a rather silly question.  I noticed last night that, having been able to plcae structures where ever I pleased for most of the game, I suddenly no longer had the option to do so.  Instead, left clicking would auto place the structure, often in a location I didn't want.

While I was trying to figure out what changed, it occured to me that this might be a feature that comes into effect your empire has grown to a certain size in order to reduce micro managment.  I personally just found it to be really annoying.

So, my question for the forums is, what caused this?  As a follow up question, how do I undo it (if possible)?

Thanks!