Changed Spelling

I've been wondering recently when the spelling of the word organization got changed to organisation?

According to the dictionary, and my almost faultless spelling ability, it is organization. But recently I have found it spelled organisation everywhere I see it. I thought maybe it was the British way of spelling it but I checked and found that Americans are misspelling it, too.

It can't be typos because it is too pervasive and consistent, and is found in documents that should have been spell-checked.