I am not sure why the Reset Settings shows in the History Panel of the Develop Module, as you already mentioned that you did not click Reset Settings.
You could reset the preferences of Lightroom once, : https://helpx.adobe.com/lightroom/kb/recover-catalog-images-resetting-lightrooms.html
I am having the same problem and came to the forum looking for help. I have two Macs both running LR CC 2015.3 and they both exhibit "Reset Settings" in the History Panel right above the Import (Date&Time) for every photo imported. Could it have to do with setting defaults for my two cameras? I recently set default Camera Profiles and Lens Correction settings - they get applied automatically as photos are imported?
Update - I just reset the defaults back to Adobe settings and tried doing an import. It didn't change the "Reset Settings" showing up in the History Panel - it still shows up after each import... so that was not what was causing it.
I 'chatted' with Adobe support today and I think we may have figured it out. Not sure it makes much sense.
I import raw files and convert to DNG and create 1:1 previews as part of my normal workflow. I also apply some default settings in Lens Corrections (Enable Profile Corrections & Remove Chromatic Aberrations) and Camera Profile (Camera Neutral). I noticed some time ago that the import process had changed. It used to be that upon import the DNG conversion happened while the images were being imported, then the previews would be built. A few months ago (?) I noticed it was working differently. The raw files were imported first then they were converted to DNG and the previews created. It appears that the Reset Settings in History applies to the DNG conversion. If I import without converting to DNG, the Reset Settings does not appear in History.
I noticed that after an import - without any further changes to the image - if I hover the mouse over the Import in History and move it back and forth over the Reset Settings and Import my 'Preview' of the image would change slightly. However if I clicked on Reset Settings and back on Import (back and forth) the main image does not change at all - nor do my settings for Lens Correction or Camera Profile. What the support person said is what I'm seeing in the Preview are the slight differences between the raw file and the DNG. But apparently, I'm guessing because the raw file does not exist any more, clicking on the Reset Settings does nothing to the main image. Which raises the question as to what good it does to have it there?
It would be nice to see some documentation, or confirmation, from Adobe on this?