This is normal behavior when you are importing photos and a preset is being applied. If you go to the history panel in your Develop Module, it should say at the bottom Import/PresetName, and you'd need to change Lightroom's importing to not apply this preset.
The other thing, besides applying a Develop preset, is that you might have the Automatically Apply Auto Tone preference enabled so check there, too.
Neither of these have fixed the problem!
In the Develop module, when I click on the photo, it shows the photo in its original form, but only for a split second, the it changes to this enhanced form. I just restored the presets in the preferences panel.
Any other suggestions?
I imported these photos a few months ago, or more, and this is a brand new problem!
I just tried the same thing, but in Lightroom 5 rather than Lightroom CC, and this auto brightening/enhancing/auto tone is not happening!