How does this happen? Well, Lightroom is very flexible environment in which to work. You can hide all sorts of things in the interface to cut down on distractions while you work. If you had selected multiple images in the grid and then pressed Shift Tab to hide all the panels followed by D to enter the Develop module you would only see the active image on the screen. If you previously had Auto Sync turned on then everything you do to that image will sync to the other selected images. If the filmstrip is hidden you might not notice until you return to the grid and see all those images adjusted! Reverting back to the initial state for those images is relatively easy. Select the images and press D to go to the Develop module. Make sure that Auto Sync is on. Now press the Reset button just to the right of Auto Sync.
That answered the reader's question but made me think about what to do if the other images weren't at their import states but had already been adjusted independently. How do you reverse adjustments without going all the way back to import state. My first thought was to bring the active image back via the History panel.
The History panel keeps track of everything you do to an image. Unlike history states in Photoshop, these steps are recorded in Lightroom's database and remain in place even if you close Lightroom. They will stay there until you either clear them or move backwards and then take another route.
Take a look at your History panel. It's on the left in the Develop module.
History steps build from the bottom up (reminiscent of a stack in the programming world). To the right of each step are two numbers. The first tells you how much of a move (positive or negative) you made for that setting. The second shows the resulting value for the setting.
Knowing this, my first thought was to simply go back in time in the active image and let Auto Sync do the rest. Unfortunately, Auto Sync doesn't respond to changes in the History panel so only the active image is changed.
Auto Sync does respond to active adjustments. Here's where the History panel can help. Remember those numbers to the right of each step? That's the key. Use the numbers to set each of the adjustments back to where they were before the adjustment was made.
As you do this the History panel will grow to reflect each of the reversals you are making. Once you're done everything should be back to the way it was. The last step is to remember to turn off Auto Sync.
Auto Sync is a wonderful and powerful tool. But, as with any tool, it's a good idea to always be aware of what state it's in and when it is being applied.