Recently while I was working on illustrator, I noticed that instead of outlining every object I selected with the same box, the top most object within the selection would have a thicker, bold bounding box. I may have bumped a button somewhere that turned this feature on.
When selecting 2 or more objects, Illustrator key selected one object for me (bold outline) and it was irritating me. I went to the top of the screen where it says "Window" then selected "Align" and the Align menu box came up. Down at the bottom of the Align menu box, click on the arrow where it says "align to" and select "align to selection". This will make your selections go back to normal. Illuatrator will auto key select an object in a multiple grouping if you have "Align to key object" selected.
The bolder highlight is to visually indicate which object has been designated as a key object for alignment. [Adobe's Help Section on Aligning] You can't "turn off" this feature. But you can simply not designate any object as a key object.
How it happens...
After an object is already selected, click it a second time to designate it as the key object.
To remove the designation, click the object a third time.
Note that this has nothing to do with the object stack or topmost object. It happens because you clicked an object twice, perhaps unintentionally.
This seems to have happened since the last update. Illustrator now automatically selects one of your selected objects as a 'key' object. Making accurately moving or transforming a large grouped object a complete pain in the... neck. It's awful and I really want there to be some way of stopping this again.