What could have changed all of the ' characters in my content to &#039 strings?

I re-ran the WordPress update to 4.0 on a backed-up version of the site (files and db) and everything went through without a hitch, so I’m not sure what happened.

I did find a plugin (Search and Replace) that helped to replace all of the resultant strings with the ascii html character that they should be. So, all in all, not too much time lost, but I’m still curious as to what happened.