Win7, 64 bit machine, Lightroom CC up to date.
With Library mode open and in grid view saw in the far right pane that the 8-year-old folder had way wrong keywords. (I guessed that I had done it... how else?)
I selected a subset of the thumbnails and changed keyword assignments.
Discovered immediately that anything typed in the keyword box was being immediately applied to the whole folder of 300 images for that day in 2007.
Copy-paste, direct entry, nothing was able to "stick" with the image(s).
I have just restarted the whole machine and LightroomCC seems to be acting normally except that NONE of the new keywords were seen left attached to the images in the folder in which I was trying to organize keywords five minutes ago. Nothing retained through the auto save process and restart. I have my catalog set for backup on every "quit session" event. In this case I think I unchecked "Optimize".
I would love to know how and why Lightroom goes all dead-circuit on me. (No disrespect intended to the robots among us...)
I recently got some great help here (thank you especially Mr. Ellis!) about AppHang81 error which was the AMD Radeon graphics card, and I had trouble with the catalog two weeks ago that required a weird step: open empty catalog and import from a back up.) I am now using an AMD beta 15.8 driver for my card, and the "Use Graphics Processor" box is unchecked in Prefs.
That's all that I can think of for triggers.
Sounds more like you had selected all the images.
I know what that looks like. As displayed, no. And I tested the oddity by
selecting one frame, (became lighter grey) changing the string, and all
immediately had the same one or set of keyword(s).