read populates $model->data

For the past couple of days, I've had this niggling doubt that I've been missing something. Some code that I had in a behavior, which was supposed to be triggering a save for two field values, further down the line kept turning into a save which included every field in the table with the old field values. Why was this happening? I asked Elvis, he didn't know so I had to suck it up and find out.