It's usually a result of the webpage doing stupid stuff to try to explicitly block password managers. There's a lot of banking and government websites that believe this makes things more secure somehow.
Several years ago I tried to register for a website that refused to accept a change of more than one character at a time, using onkeyup and other events to ensure you couldn't fill it in any way other than one letter at a time (fortunately this was only on the asking setup page; it works fine with logging in). After every change in value, it compared the current length to the previous length, and rejected it if the difference was more than one.
